Internalizing a key in the KeyedStoreICGeneric avoids an expensive SetProperty runtime call. 
This improves the prepack benchmark by ~5%. 
In the micro-benchmark copy-object.js attached to the bug, it surfaces as a ~2.5x improvement.
The performance improvement currently relies on the stub cache, since we don't search for 
transitions from within the CSA. As this CL puts additional stress on the stub cache, 
performance regressions wouldn't be too surprising.

Bug: v8:6936, v8:6997
Change-Id: Id1469499a3ae5450519ff40d3c5a0915c6de0d45
Reviewed-on: https://chromium-review.googlesource.com/749951Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49097}