This CL defines move semantics for String16, and fixes issues with the
hash code not being set correctly on swap or copy.
It also extends the interface by a few handy templates.
All this functionality will be used for the wasm translations, where
String16s are often concatenated and used as keys in hash tables.

BUG=chromium:659715
R=yangguo@chromium.org, kozyatinskiy@chromium.org

Review-Url: https://codereview.chromium.org/2493723003
Cr-Commit-Position: refs/heads/master@{#41007}