Increase default DevTools request timeout to 30 seconds.

The timeout was recently changed from None (wait forever) to 5 seconds.
On slow bots, even after _WaitForBrowserToComeUp returns, the launch of
the first renderer can take considerable time. The next request for the
browser's state is subject to the timeout here, and the browser does not
respond until the renderer's launch completes.

Increasing the timeout to 30 seconds should not regress the original fix
and should fix the flakiness.

BUG=403981
TBR=tonyg@chromium.org

Review URL: https://codereview.chromium.org/474413002

Cr-Commit-Position: refs/heads/master@{#290133}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290133 0039d316-1c4b-4281-b951-d872f2087c98

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

@@ -194,7 +194,7 @@ class ChromeBrowserBackend(browser_backend.BrowserBackend):
   def ListInspectableContexts(self):
     return json.loads(self.Request(''))
 
-  def Request(self, path, timeout=5, throw_network_exception=False):
+  def Request(self, path, timeout=30, throw_network_exception=False):
     url = 'http://127.0.0.1:%i/json' % self._port
     if path:
       url += '/' + path