This turns on the existing --internalize_on_the_fly flag for the
MEGAMORPHIC KeyedLoadIC to properly internalize strings before
looking up the property. This avoids the otherwise taken runtime
call to %KeyedGetProperty, which is definitely slower.

Initially the --internalize_on_the_fly flag was turned off because
internalizing strings on the fly causes too much traffic on the
megamorphic stub cache. We avoid this problem here by not probing
the stub cache in that case, which still gives the benefit of not
having to go to the runtime.

This improves the babylon test on the web-tooling-benchmark by around
2-3% and will probably also help with several tests (like React or
Ember) on the Speedometer benchmark.

If this CL causes trouble (i.e. tanks something important), we can
just turn off the --internalize_on_the_fly flag again.

Bug: v8:6936, v8:7026
Change-Id: If295ed3fd013f8b0ff031f9979e7df21dab817b6
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/751464Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49093}