Remove KeyframeEffectModel.h include from StyleResolver.h

Replace the include with forward declarations.

Prior to this patch, KeyframeEffectModel.h had roughly 700 targets
depending on it. After this patch this number has shrunk to around 11.
In the last 90 days, KeyframeEffectModel.h was modified 13 times on
trunk, which is a relatively high number.

KeyframeEffectModel.h also pulls in a large amount of other animation
headers so they should see similar benefits.

Review URL: https://codereview.chromium.org/346803002

git-svn-id: svn://svn.chromium.org/blink/trunk@176517 bbb929c8-8fbe-4397-9dbb-9b2b20218538