GitLab

This CL moves the call to
  canonical_mime_type_ =
      network::CrossOriginReadBlocking::GetCanonicalMimeType(mime_type);
closer to the point where the value of the |canonical_mime_type_| field
is needed for the 1st time.

The move helps with the following:
- Makes unit tests more robust against shuffling of chucks inside
  ShouldBlockBasedOnHeaders (some shuffling will be needed when
  moving some checks into //services/network).
- Makes the code of ShouldBlockBasedOnHeaders closer to the promise to
  perform less expensive checks first (the GetCanonicalMimeType has
  medium cost - it has to compare the mime type against multiple
  hardcoded strings).

The move necessitates some small follow-up tweaks in unit tests.

Bug: 792546
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: Ib21add69443dd0748aafd57da89a3cc16ffcdec1
Reviewed-on: https://chromium-review.googlesource.com/957804Reviewed-by: Nick Carter <nick@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542643}