Make content::TitleWatcher robust to fast-running tests

Tests that use TitleWatcher usually have the use pattern:

1. Test harness runs test (usually JS in a page).
2. Test harness waits for window title to change using TitleWatcher.
3. Test execution completes and changes window title to notify harness.
4. TitleWatcher is notified of window title and unblocks test harness.

In the case that the test execution runs fast enough to change the window title
before the TitleWatcher begins its wait, the title change is never noticed and
the test appears to hang/timeout.  Fix this.

BUG=341265
TEST=local build, run unittests on desktop Linux

Review URL: https://codereview.chromium.org/156363002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249575 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249575