Switch to a single merged version of WebThemeEngine.h

instead of using multiple platform-specific versions; the latter
made it confusing to know which files to include and what was safe.

Ultimately we still need to merge this further into a single consistent cross-platform API, though.

R=jamesr@chromium.org
BUG=327471

Review URL: https://codereview.chromium.org/154733004

git-svn-id: svn://svn.chromium.org/blink/trunk@166988 bbb929c8-8fbe-4397-9dbb-9b2b20218538