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Design doc: https://bit.ly/browser-issue-storage

This CL adds an Issue storage in the browser process, so Issues
no longer need to be sent to the renderer process to be stored and
reported there.

Characteristics of the storage:
  - The issue count is capped at 1000 issues per WebContent
  - The storage is cleared for main frame navigations (modulo redirects)
  - When subframes get deleted, issues associated with the deleted
    subframe get "re-parented" to the main frame

Bug: chromium:1063007
Change-Id: I75f164812770e8c73fc388d27767cc9151e5e4af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2246139
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781237}