Remove the context specialization hack from the AstGraphBuilder, and
properly specialize to the function context in the context specialization.
And replace the correct context in the JSInliner.

R=mstarzinger@chromium.org
BUG=v8:4273
LOG=n

Review URL: https://codereview.chromium.org/1218873005

Cr-Commit-Position: refs/heads/master@{#29493}