mac: Fix a history swiper bug where the shield wouldn't disappear.

There were 3 separate problems.
1. Retain bug :(
2. historyOverlay was removing the view from the superview during dealloc,
which is atypical. If the view didn't exist, historyOverlay was actually
creating the view, and sending a callback to the delegate.
3. We were not specifying an NSAnimationContext grouping/duration, which
non-deterministically would use a 0-duration animation, which actually
prevented the animation and callback from triggering. As a result, the
shield would never disappear.

On a side note: unit_tests did not work when run with
--gtest-filter=HistoryOverlay* because we did not specify an NSAnimationContext
with a non-zero duration. 0-duration animation contexts cause animations to not
fire, which meant that the delegate callback never went through.

BUG=330312
TEST=manual

Review URL: https://codereview.chromium.org/107343008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242911 0039d316-1c4b-4281-b951-d872f2087c98
Remove OVERRIDEs from WebSourceBufferImpl

Removes OVERRIDEs for this Chromium implementation of a Blink interface.
Discussion revealed this is allowed (and preferred), to reduce cross-repo API change difficulties.
Also removes TODOs related to some pending OVERRIDE additions (see the referenced bugs). Though the Blink changes have landed, OVERRIDE is undesired.

R=acolwell@chromium.org
BUG=166465,229408

Review URL: https://codereview.chromium.org/123483002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242912 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242912