WebRTC browser tests now detect javascript errors.

The manual WebRTC browser tests will now fail when there's a top-level
javascript error, such as uncaught exceptions. This should help keep
the test javascript clean.

The detection mechanism assumes the tests run sequentially, which is
why I have only modified the manual tests which will always run
sequentially (and not, for instance, the infobar tests).

This CL also simplifies the error handling and does away with "stored" errors.

BUG=307397

Review URL: https://codereview.chromium.org/104493003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239471 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239471