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As the last step to convert ViewMsg_UpdateWebPreferences
IPC message to Mojo, this CL moves ViewMsg_UpdateWebPreferences
to PageBroacast Mojo interface.

  - Introduce web_preferences.mojom which has WebPreferences
    Mojo struct with needed enum types.
  - Add web_preferences_mojom_traits.h/cc files to map Mojo fields
    to the proper native types.
  - Remove IPC legacy stuff regarding the IPC message in //content.

Bug: 1097943
Change-Id: I3a4286b106b7f0a0b38cb7482f1f0f8f7fbf590f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426070Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#812602}