Remove legacy StartsWithASCII function.

This replaces it with base::StartsWith and the appropriate case flag. Since the existing version only ever did case-insensitive tests in ASCII, there should be no behavior change.

BUG=506255
TBR=jam

Review URL: https://codereview.chromium.org/1242023005

Cr-Original-Commit-Position: refs/heads/master@{#339175}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 955093110ad64d5ec6f5426991efaa4a680b5d6f