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ParkableStrings can be written out to disk, through DiskDataAllocator
which multiplexes all writes onto a single file descriptor. The file is
provided by the browser process.

This adds the logic on the browser side to actually provide the file. It
is guarded by a (disabled by default) feature, and in any case for
incognito renderers.

This CL adds:
- A mojo interface to provide the file
- A feature to enable field trials
- Plumbing on the browser and renderer side.

Bug: 1029320
Change-Id: I6d1b4ce72561e6ac4dd74a1c4d2fd60a23c48995
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2160912
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761903}