Add -wait-for-browser for Linux and macOS as well. (#25658)

In previous PR #25644
I added `-wait-for-browser` use on Windows to help single-threaded
Windows runner not leave behind stale browser windows.

For some reason, it was not passing on CircleCI on Linux. Testing this
again to see what the failure is.

In local testing with a test script:

```py
import subprocess, sys, time

cmd = ['C:\\Program Files\\Mozilla Firefox\\firefox.exe', '-new-instance', '-profile', sys.argv[1], '-wait-for-browser', 'https://wiki.mozilla.org/Firefox/CommandLineOptions']
print('Launching browser: ' + ' '.join(cmd))
proc = subprocess.Popen(cmd)

for i in range(10):
  if proc.poll() is not None:
    print('Oops: process already quit, so we did not get a process handle to keep track of the browser instance.')
    sys.exit(0)
  time.sleep(1)

if proc.poll() is None:
  print('terminating browser process')
  proc.terminate()
time.sleep(2)
if proc.poll() is None:
  print('terminating did not work. killing browser process')
  proc.kill()

print('All done')
```

I am unable to find a behavioral difference between Windows and Linux.

So testing this out again on CircleCI.

Author: juj

test/common.py

@@ -134,17 +134,29 @@ class ChromeConfig:
   def configure(data_dir):
     """Chrome has no special configuration step."""
 
+  @staticmethod
+  def open_url_args(url):
+    return [url]
+
 
 class FirefoxConfig:
   data_dir_flag = '-profile '
-  default_flags = ('-new-instance', '-wait-for-browser') if WINDOWS else ('-new-instance',)
+  default_flags = ('-new-instance', '-wait-for-browser')
   headless_flags = '-headless'
   executable_name = utils.exe_suffix('firefox')
 
   @staticmethod
   def configure(data_dir):
     shutil.copy(test_file('firefox_user.js'), os.path.join(data_dir, 'user.js'))
 
+  @staticmethod
+  def open_url_args(url):
+    # Firefox is able to launch URLs by passing them as positional arguments,
+    # but not when the -wait-for-browser flag is in use (which we need to be
+    # able to track browser liveness). So explicitly use -url option parameter
+    # to specify the page to launch. https://bugzil.la/1996614
+    return ['-url', url]
+
 
 class SafariConfig:
   default_flags = ('', )
@@ -160,6 +172,10 @@ class SafariConfig:
   def configure(data_dir):
     """ Safari has no special configuration step."""
 
+  @staticmethod
+  def open_url_args(url):
+    return [url]
+
 
 # Special value for passing to assert_returncode which means we expect that program
 # to fail with non-zero return code, but we don't care about specifically which one.
@@ -2059,7 +2075,7 @@ def browser_open(cls, url):
     if (WINDOWS and is_firefox()) or is_safari():
       cls.launch_browser_harness_with_proc_snapshot_workaround(parallel_harness, config, browser_args, url)
     else:
-      cls.browser_procs = [subprocess.Popen(browser_args + [url])]
+      cls.browser_procs = [subprocess.Popen(browser_args + config.open_url_args(url))]
 
   @classmethod
   def launch_browser_harness_with_proc_snapshot_workaround(cls, parallel_harness, config, browser_args, url):
@@ -2076,7 +2092,7 @@ def launch_browser_harness_with_proc_snapshot_workaround(cls, parallel_harness,
         procs_before = list_processes_by_name(config.executable_name)
 
       # Browser launch
-      cls.browser_procs = [subprocess.Popen(browser_args + [url])]
+      cls.browser_procs = [subprocess.Popen(browser_args + config.open_url_args(url))]
 
       # Give the browser time to spawn its subprocesses. Use an increasing
       # timeout as a crude way to account for system load.