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This CL introduces ResourceSnapshotForWebBundle mojo interface and
GetResourceSnapshotForWebBundle() method in LocalFrame mojo interface.
This method will be used to create a web bundle file.
Design Doc: https://docs.google.com/document/d/1O1fH7ev2NwRqOVd_WjEtv4JaI6fA4x6G3ogfz6Hi9Pc/edit#
This is the working-in-progress CL: https://crrev.com/c/1994848
  - The browser process will call GetResourceSnapshotForWebBundle()
    to get the Mojo endpoint of the interface.
  - The browser process will pass the Mojo endpoint to the utility
    (data decoder) process.
  - The utility process will calls the methods of the interface using
    the passed Mojo endpoint, and will generate a Web Bundle file.

Basically this CL implements GetResourceSnapshotForWebBundle() method
using the existing page serialization logic for MHTML generation.
For web bundle generation, we need to change the serialization logic:
  - Subframe URL replacement
  - Style element URL replacement
  - Shadow DOM support
But this CL doesn't implement the web bundle specific logic.
We will create another CL to implement them.

Bug: 1040752
Change-Id: Ia7470de2cd1e418d8bc2d393f5136042145f62d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2060360Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746265}