Use StringView for startsWith and endsWith.

This lets us collapse lots of the code paths together, and means we can
now handle startsWith(kConst) without doing a strlen or a malloc of a
String object.

BUG=615174

Review-Url: https://codereview.chromium.org/2146163003
Cr-Commit-Position: refs/heads/master@{#405410}
6 files changed