There are no ICs left that store their state in this field: vector based
ICs use feedback vector and the rest three (BinaryOpIC, CompareIC and
ToBooleanIC) reconstruct their state from the ExtraICState field.

This CL also removes unused InlineCacheState::DEBUG_STUB which was used
mostly in Code::is_debug_stub(). The latter now checks if the code is one
of the debug builtins instead.

BUG=chromium:618701
LOG=Y

Review-Url: https://codereview.chromium.org/2052763003
Cr-Commit-Position: refs/heads/master@{#36871}