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
R=jamesr

Take 2 in landing after fixing an incorrect include path in
ScrollbarThemeAndroid.cpp.

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


Review URL: https://codereview.chromium.org/23087002

git-svn-id: svn://svn.chromium.org/blink/trunk@156033 bbb929c8-8fbe-4397-9dbb-9b2b20218538