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When WebRTC textual log files are deleted, either because of
expiration or because the user clears browsing history,
entries from the logs' index are now also removed.

As a drive-by:
* Fixed the unbounded reading of a file into memory.
* Marked with TODOs (and filed bugs) for several other
  problems found.

Bug: 825977, 827131
Change-Id: If34acf6290ec90c3ce99c0704c58bc8a8188f92d
Reviewed-on: https://chromium-review.googlesource.com/982055Reviewed-by: Henrik Grunell <grunell@chromium.org>
Commit-Queue: Elad Alon <eladalon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548455}