blink: minor tweak to fast path comparing end tag names

When comparing end tags there is no need to call ScanTagName()
as we know exactly what we are looking for. This changes the
code to compare the string contents directly and if that fails
try comparing with mapping to lower case.

Change-Id: I6366541a6c38a87def1a2caf31430f63622e73c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4505604


Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147376}