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Implements DidReceiveData() to save data received from the
blink::WebAssociatedURLLoader, and ReadResponseBody() to return the data
to the caller.

Based largely on whatever
ppapi::proxy::URLLoaderResource::ReadResponseBody() does.

Bug: 1099022
Change-Id: Ibaa7db0a7af5996e153d8dc106066ca6133ba95c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2407175Reviewed-by: K. Moon <kmoon@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Commit-Queue: K. Moon <kmoon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807347}