Port 13fc4a1b

Original Commit Message:

    External references located within the isolate can be loaded as a
    fixed offset from the root register. This avoids one load from memory
    over the default indirect method (which reads from the external
    reference table).

R=jgruber@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: I4a02973a0279517ab5247f4f9519b0e26168c444
Reviewed-on: https://chromium-review.googlesource.com/1097628Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#53680}