String::SlowFlatten assumed that ConsStrings with empty first parts have
flattened strings as their second part. TurboFan, however, can create
ConsStrings with empty first parts and arbitrary second parts. With
this CL we call String::Flatten on the second part of a ConsString if
the first part is empty, but only when String::Flatten would not call
String::SlowFlatten.

R=jkummerow@chromium.org

BUG=chromium:696651

Change-Id: I9acb681de1be695e1ec2f6f6d28b9e4dc4344e98
Reviewed-on: https://chromium-review.googlesource.com/448457
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43513}