Consolidate WebInputEvent coalescing logic

Code for coalescing the various WebInputEvent types is littered throughout the
code.  Move all such logic into WebInputEventTraits, making possible standalone
test for event coalescing, and simplifying code that relies on coalescing.

BUG=303238

Review URL: https://codereview.chromium.org/26923002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228828 0039d316-1c4b-4281-b951-d872f2087c98
Remove NOTIFICATION_WEB_CONTENTS_DESTROYED from ContentSettingBubbleContents.

BUG=170921
TEST=everything still works

Review URL: https://codereview.chromium.org/27230003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228829 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228829