Update to the version of googletest current in chromium.  Googlemock has
merged into googletest so we get rid of the separate DEPs entry and point
everything inside googletest.

We put it in 'third_party' instead of 'testing' to prepare for the
upcoming stand-alone gn build which borrows chromium build files that
require other stuff in the 'testing' directory.

BUG=angleproject:1569

Change-Id: I51d1675efd5d0104bb6390721928010221a5d6fc
Reviewed-on: https://chromium-review.googlesource.com/713466Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>