Merge Settings::useOverlayScrollbars() into RuntimeEnabledFeatures::overlayScrollbarsEnabled()

Settings::useOverlayScrollbars is a global setting (not per Settings object)
that is only set to true during some layout tests. We can reuse
RuntimeEnabledFeatures::overlayScrollbarsEnabled for this rather than
having 2 separate global flags that do the same thing.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/23513028

git-svn-id: svn://svn.chromium.org/blink/trunk@157561 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Rename Custom Elements entered, left document callbacks.

These now refer to 'view', not 'document'.

BUG=234509

Review URL: https://chromiumcodereview.appspot.com/23660007

git-svn-id: svn://svn.chromium.org/blink/trunk@157562 bbb929c8-8fbe-4397-9dbb-9b2b20218538