Remove use of GetBlockingPool from media galleries.

This also removes some usage of the FILE thread as well, but not all.

Bug: 675631
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_dbg_ng
Change-Id: I97dc1b3bbe52f865ccbaf639bd33fc5dd65b474b
Reviewed-on: https://chromium-review.googlesource.com/566744
Commit-Queue: Ben Wells <benwells@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Francois Doray <fdoray@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#498359}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e4a2323376aaac1259084ff92cddcbabb32da442

check_gn_headers_whitelist.txt

@@ -57,7 +57,6 @@ chrome/browser/media/router/mojo/media_route_provider_util_win.h
 chrome/browser/media/webrtc/desktop_media_list_ash.h
 chrome/browser/media/webrtc/desktop_media_list_observer.h
 chrome/browser/media/webrtc/rtp_dump_type.h
-chrome/browser/media_galleries/fileapi/file_path_watcher_util.h
 chrome/browser/media_galleries/fileapi/iapps_data_provider.h
 chrome/browser/media_galleries/fileapi/itunes_data_provider.h
 chrome/browser/media_galleries/fileapi/picasa_data_provider.h