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This CL adds a singleton LearningSession to BrowserContext. This
provides a single location from which to get LearningTaskControllers,
which will allow aggregating observations accross multiple frames.

Bug: 994831
Change-Id: Ibeaf47a09ae7408d0afab9bc113a98e9f1532199
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758625Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688733}