The StringEqual builtin can now deal with ExternalOneByteStrings
without going to the runtime. It still needs to go to the runtime
if the external string is short, i.e. if the resource data pointer
is not cached.

R=yangguo@chromium.org
BUG=v8:4913

Review-Url: https://codereview.chromium.org/2690853004
Cr-Commit-Position: refs/heads/master@{#43152}