Port 3518e492

Original commit message:

    Short external strings do not cache the resource data, and may be used
    for compressible strings. The assumptions about their lengths is
    invalid and may lead to oob reads.

R=yangguo@chromium.org, joransiu@ca.ibm.com, bjaideep@ca.ibm.com, michael_dawson@ca.ibm.com, mbrandy@us.ibm.com

BUG=v8:4923,chromium:604897
LOG=N

Review URL: https://codereview.chromium.org/1911633002

Cr-Commit-Position: refs/heads/master@{#35682}