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This is a part of moving manifest implementation to blink.
This CL moves content::ManifestFetcher to blink and introduces
WebManifestFetcher in blink/public/web to expose the fetcher to
content::ManifestManager.

Bug: 704441
Change-Id: Ia4114390f124973542aca0151d06baec82fdaec1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1548972Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Cr-Commit-Position: refs/heads/master@{#649147}