network error: Save easter egg highscore to profile

Save the dino game highscore into a preference in the user's profile.
The highscore will persist across sessions and sync across devices.

Adds a mojo interface that allows setting the highscore preference from
the renderer. NetErrorPageController / NetErrorHelper expose new
functionality to provide the saved score to the error page and update
the saved score when a new highscore is reached.

Also ensures that the maximum high score is chosen on sync conflicts by
extending PrefModelAssociatorClient to allow the embedder to choose the
merging strategy for conflicting preference values.

TBR=sdefresne@chromium.org

Bug: 568093
Change-Id: Ia6485cbbf03ea210cba11dd3bd5f8be2518997d7
Reviewed-on: https://chromium-review.googlesource.com/c/1347283
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Edward Jung <edwardjung@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611711}
24 files changed