mac: Fix a newly introduced history swiping bug.

Attempting to 2-finger history swipe twice in quick succession would
fail to register the second swipe ~50% of the time. The NSEvent callback with
phase=NSEventPhaseBegan non-deterministically occurs before the event has been
registered as a gesture. This was causing the history_swiper to process the
event as part of the previous gesture.

BUG=138175
TEST=In a single tab, navigate to 5 different sites. Use a trackpad to 2-finger
swipe repeatedly in the same direction. Each swipe should register as a history
swipe, and cause the page to navigate in the appropriate direction. There
should be no jankiness, and the touches should not fail to register.

Review URL: https://codereview.chromium.org/117733002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241918 0039d316-1c4b-4281-b951-d872f2087c98
[MemSheriff] Suppress V8PerContextDataHolder leak

Occasionally happens in WebWorkers

NOTRY=true
BUG=329960
TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/119423002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241919 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241918
SVN changes up to revision 241919