cros: Fix EventRouter shutdown crash due to AccessibilityManager observer

The AccessibilityManager wasn't unregistering itself as an observer and it is
deleted before Profile/EventRouter deletion.

This is a temporary fix until https://codereview.chromium.org/82553013/ can
land.

BUG=322793
TEST=browser_tests
R=dmazzoni@chromium.org

Review URL: https://codereview.chromium.org/88723002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237352 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237352
Round deltas for mouse-based synthetic scroll.

Even though WebMouseWheelEvents take floating point deltas, internally the
scroll position is stored as an integer. Rounding the deltas ensures that
the gesture state, especially |current_y_|, is consistent with the internal
state.

Re-enable ScrollActionTest, as this should fix flakiness in testScrollAction
unit test.

BUG=323249
NOTRY=true

Review URL: https://codereview.chromium.org/88713002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237353 0039d316-1c4b-4281-b951-d872f2087c98
Add prerender histograms.

R=asvitkine@chromium.org

Review URL: https://codereview.chromium.org/88253003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237354 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237354