[Cleanup] Remove unnecessary including GraphicsContext.h in GraphicsLayer.cpp

We don't need to include GraphicsContext.h in GraphicsLayer.cpp because
no one references the api of GraphicsContext.

R=esprehn@chromium.org
BUG=NONE
TEST=Compiles

Review URL: https://codereview.chromium.org/119673003

git-svn-id: svn://svn.chromium.org/blink/trunk@164345 bbb929c8-8fbe-4397-9dbb-9b2b20218538