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Create Notifier class to pass messages to observers, which will lead to
HTTP responses and requests to be displayed in the HTTP Messages
tab of the internal debug page.

Bug: 1093634
Change-Id: Iafcc02a1bcfb3e89085ab60ed35d16cecc312e79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2299521
Commit-Queue: Juliet Levesque <julietlevesque@google.com>
Reviewed-by: Josh Nohle <nohle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791536}