The ManifestManager interface is channel-associated to preserve ordering
only in tests.

The manifest browser tests rely on console logging IPCs being delivered
FIFO with the ManifestManager::RequestManifest response.  Replace this
channel-association with an explicit wait for a channel-associated round
trip only in the test by creating a channel-associated interface and
using FlushForTesting().

The service worker manifest fetching tests rely on injected JS code
running before the manifest request is received by the renderer. Change
this to wait for a response before requesting the manifest.

Bug: 704441
Change-Id: I290a72125769aae09b5460b75dbb2e0cbfe46c1d
Reviewed-on: https://chromium-review.googlesource.com/746503


Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513342}