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=bmeurer@chromium.org
BUG=v8:4923,chromium:604897
LOG=N

Review URL: https://codereview.chromium.org/1902393004

Cr-Commit-Position: refs/heads/master@{#35683}