Remove virtual destructors for CustomElementDefinition and CustomElementRegistry

Remove destructors for CustomElementDefinition and CustomElementRegistry as these
were empty and marked as virtual. This avoids creating unnecessary vtables since
these classes are never subclasses nor do they have any virtual function.

Review URL: https://codereview.chromium.org/41133002

git-svn-id: svn://svn.chromium.org/blink/trunk@160505 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Disable ScrollingCoordinatorChromiumTest.rtlIframe on Windows

BUG=311425
TBR=falken@chromium.org

Review URL: https://codereview.chromium.org/42683002

git-svn-id: svn://svn.chromium.org/blink/trunk@160506 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Mark png-suite/test.html tests as flaky on Mac

BUG=311429
TBR=falken@chromiun.org

Review URL: https://codereview.chromium.org/41703004

git-svn-id: svn://svn.chromium.org/blink/trunk@160507 bbb929c8-8fbe-4397-9dbb-9b2b20218538