Reland r251495: Re-enable prerender RemovingLink browser tests.

The tests have changed significantly since they were first disabled. Add a
WaitForStop or two for good measure, but leave them as-is for the most part.
They can be disabled again if they still flake. Merge their custom prerender
loader into the main one; it's mostly the same.

In addition, for better test coverage, add a new test which asserts on events
received when a <link rel=prerender> is added for an existing prerender after
that prerender has loaded. Significantly rework the prerender events logic
to allow the test framework to wait on an event being received in the loader.

Fix implementation of set_loader_query_and_fragment to not produce URLs with
two ?s.

Original Review URL: https://codereview.chromium.org/142013004

BUG=167340, 128841

Review URL: https://codereview.chromium.org/166273007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252156 0039d316-1c4b-4281-b951-d872f2087c98
Fixes hover over keys changing cursor icon.

BUG=344943

Review URL: https://codereview.chromium.org/172263003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252157 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252157