Refactor paintOverhangAreas to allow non-Mac Chromium platforms to reuse code

Refactor paintOverhangAreas to allow non-Mac Chromium platforms to reuse code
https://bugs.webkit.org/show_bug.cgi?id=68648

Reviewed by Dimitri Glazkov.

Source/WebCore:

No new tests because there's no change in functionality (yet).

* platform/chromium/ScrollbarThemeChromium.cpp:
(WebCore::ScrollbarThemeChromium::ScrollbarThemeChromium):
(WebCore::ScrollbarThemeChromium::~ScrollbarThemeChromium):
(WebCore::ScrollbarThemeChromium::paintOverhangAreas):
* platform/chromium/ScrollbarThemeChromium.h:
* platform/chromium/ScrollbarThemeChromiumMac.h:
* platform/chromium/ScrollbarThemeChromiumMac.mm:
(WebCore::ScrollbarThemeChromiumMac::ScrollbarThemeChromiumMac):

Source/WebKit/chromium:

* features.gypi:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@95860 268f45cc-cd09-0410-ab3c-d52691b4dbfc