Generate thread-safe Event interface names

This CL generates thread-safe strings for each Event interface name. This CL
will let us delete the remainder of the ThreadLocalEventNames.

R=eseidel

Review URL: https://codereview.chromium.org/26563005

git-svn-id: svn://svn.chromium.org/blink/trunk@159367 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove layerFor*Scrollbar and layerForScrollCorner

These functions on RenderLayer were only used by Internals
which can access them through the ScrollableArea.

BUG=260899

Review URL: https://codereview.chromium.org/26804005

git-svn-id: svn://svn.chromium.org/blink/trunk@159368 bbb929c8-8fbe-4397-9dbb-9b2b20218538