Fix ScopedBlockingCall DCHECK when analyzing DMG files.

file_analyzer.cc opens dmg on the UI thread which disallows blocking leading
to a DCHECK failure on debug builds.

Test:

1. Open chromium (debug)
2. Navigate to
   https://github.com/MagerValp/AutoDMG/releases/download/v1.9/AutoDMG-1.9.dmg
3. No crash

Bug: 921148
Change-Id: Idb3a06d747a61ed5f92c5f3be92d791877dcd3c7
Reviewed-on: https://chromium-review.googlesource.com/c/1414131
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623804}
5 files changed