Move manifest_util.cc/.h & unittest in content to blink.

This is a part of moving manifest implementation to blink.
This is the precursor patch before moving content::ManifestParser
to blink.

Bug: 704441
Change-Id: I1cf7f277f859aaaa7a2e031ab9dba56e74a293cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1527951
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Miyoung Shin <myid.shin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642718}
10 files changed