[Telemetry] Increase timeout for HasBrowserFinishedLaunching

Empirically it can take chrome up to 40 seconds to come up for a cold start on the 10.6 bots. Increase the timeout in Telemetry to reduce test flakiness.

BUG=405386

Review URL: https://codereview.chromium.org/568673004

Cr-Commit-Position: refs/heads/master@{#295248}

tools/telemetry/telemetry/core/backends/chrome/chrome_browser_backend.py

@@ -144,7 +144,7 @@ class ChromeBrowserBackend(browser_backend.BrowserBackend):
 
   def _WaitForBrowserToComeUp(self, wait_for_extensions=True):
     try:
-      util.WaitFor(self.HasBrowserFinishedLaunching, timeout=30)
+      util.WaitFor(self.HasBrowserFinishedLaunching, timeout=50)
     except (util.TimeoutException, exceptions.ProcessGoneException) as e:
       if not self.IsBrowserRunning():
         raise exceptions.BrowserGoneException(self.browser, e)