Oilpan: remove consts when checking GC-class convertibility.

Achieve the desired conversion outcomes when working with const types,
explicitly removing "const" qualifiers throughout before testing if
class types can be converted to GarbageCollected{Mixin,Finalized} etc.

R=haraken@chromium.org,erik.corry@gmail.com
BUG=

Review URL: https://codereview.chromium.org/282263006

git-svn-id: svn://svn.chromium.org/blink/trunk@174175 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Added missing null check to avoid crash.

Added missing null check for RenderLayerScrollableArea to avoid crash.
This is a fix done by code inspection. That's why we have no test.

Review URL: https://codereview.chromium.org/288143002

git-svn-id: svn://svn.chromium.org/blink/trunk@174176 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Auto-rebaseline for r174060

http://src.chromium.org/viewvc/blink?view=revision&revision=174060

BUG=369609
TBR=enne@chromium.org

Review URL: https://codereview.chromium.org/287153005

git-svn-id: svn://svn.chromium.org/blink/trunk@174177 bbb929c8-8fbe-4397-9dbb-9b2b20218538