Fix some by removing (unnecessarily) implicitly declared constructors
and assignment operators.
String16 constructors and assignment operators can just be defaulted,
and declared in the header.

This fixes the last complaints of the new presubmit check.

R=mlippautz@chromium.org, yangguo@chromium.org, leszeks@chromium.org

Bug: v8:8616
Change-Id: Idae7031b88b793253b63488c52f757513711ed73
Reviewed-on: https://chromium-review.googlesource.com/c/1417173Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59004}