[chromium] Put webcore_platform_files in separate gyp target instead of relying on...

[chromium] Put webcore_platform_files in separate gyp target instead of relying on exclusion patterns
https://bugs.webkit.org/show_bug.cgi?id=95876

Reviewed by Tony Chang.

This puts the list of WebCore/platform files in a webcore_platform_files gyp variable instead of filtering by
path out of webcore_files. This simplifies the .gyp a bit and makes it possible to split this target up further.

* WebCore.gyp/WebCore.gyp:
* WebCore.gypi:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@127629 268f45cc-cd09-0410-ab3c-d52691b4dbfc