When running with FLAG_runtime_call_stats, native accessor accesses must
go through the runtime for accurate accounting. Previously the slow_stub()
was used as a handler in order to accomplish this, but it could never be
looked up from the code cache successfully due to mismatched code flags,
which could cause more handler recompilations than in normal operation.
This patch fixes that by emitting a runtime call into the compiled
handler instead of using the slow_stub().

Drive-by cleanup: drop the unused StoreIC_Megamorphic builtin.

Review-Url: https://codereview.chromium.org/2054133002
Cr-Commit-Position: refs/heads/master@{#36926}