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-Original-Commit-Position: refs/heads/master@{#405410}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a5673d69964d773f0111e2e6a600ac180587c2f6