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Implements a state machine to bridge the gap between
chrome_pdf::UrlLoader's pull model and blink::WebAssociatedURLLoader's
push model.

This change only handles the basic state transitions; this is sufficient
to simulate an empty response. A future change will deal with handling
the data received by DidReceiveData().

Also fills in a basic blink::WebURLRequest, since BlinkUrlLoader calls
WebAssociatedLoader::LoadAsynchronously() now. Note that RequestContext
and RequestDestination must be set to pass certain security checks.

Bug: 1099022
Change-Id: I20840e44adda63816f8c7d6a531ffe106dd8bfdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2411547
Commit-Queue: K. Moon <kmoon@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807345}