Add WebURLLoaderMockFactory::GetLastHandledAsynchronousRequest.

This provides a way of getting the last request that was actually (mock) served,
so that WebKit unit tests can, e.g., inspect headers that were "actually" sent
out.

BUG=http://crbug.com/134615 and https://bugs.webkit.org/show_bug.cgi?id=90893
TEST=see WebKit bug (forthcoming WebKit unit test)

Review URL: https://chromiumcodereview.appspot.com/10735037

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@145960 0039d316-1c4b-4281-b951-d872f2087c98
4 files changed