GitLab

Adds OnFileModified observer interface to FileChangeServiceObserver
called when the file is written, truncated, or copied to.

Fixes file change service unit tests to verify that mocked observer
methods actually get called. The observers were previously not
getting invoked because the test suite did not set up fake chrome
user manager, so user account IDs were not correctly mapped to the
associated test profile, which is something
ObservableFileSystemOperationImpl depends on to get the
FileChangeService to notify of file changes.

BUG=1139115

Change-Id: I9829db0a496eec301a7f7547ac74f61bc1a18bdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2603467Reviewed-by: Sergei Datsenko <dats@chromium.org>
Reviewed-by: David Black <dmblack@google.com>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839697}