Cleanup the ScrollbarTheme classes.

1) Make all filenames consistent with their filenames.
2) Move all files into the core/platform directory out of the
chromium and mac directories.
3) Remove Chromium from filenames.
4) Draw an inheritance diagram in ScrollbarTheme.h since it's complicated
and there are so many files.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/22754004

git-svn-id: svn://svn.chromium.org/blink/trunk@155974 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove unneeded const on updateOffsetIfNeeded method.

Review URL: https://chromiumcodereview.appspot.com/22909005

git-svn-id: svn://svn.chromium.org/blink/trunk@155975 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Web Animations CSS: Fix crash when animation is removed after it has finished

When an animation-name: rule is removed we look up and attempt to cancel
a player for that animation. However, DocumentTimline currently removes
and derefs Players when it detects that they are finished. Holding a raw
pointer to Players from CSSAnimations is not sufficient.

BUG=258896

Review URL: https://chromiumcodereview.appspot.com/22961003

git-svn-id: svn://svn.chromium.org/blink/trunk@155976 bbb929c8-8fbe-4397-9dbb-9b2b20218538