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This change introduces core concepts of Recent file system:

RecentModel:

  The most important class owning RecentSources.

  The only API it provides is GetRecentFiles() which queries
  RecentSources for recently modified files and build a list of
  recently modified files across file systems, and also caches the
  result for better file system operation performance.

  Files are represented as FileSystemURL.

RecentSource:

  Interface class for a source of recent files.

  This interface will be implemented for several sources such as local
  storages and cloud storages so that they appear in Recent file
  system.

RecentContext:

  Holds several "context" objects necessary for operations in
  RecentModel and RecentSource.

Bug: 742722
Test: unit_tests
Change-Id: I2a2ff1fa28e8cf5c705ad1ee6bc75a39b49b2f0b
Reviewed-on: https://chromium-review.googlesource.com/588950Reviewed-by: Tomasz Mikolajewski <mtomasz@chromium.org>
Commit-Queue: Shuhei Takahashi <nya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491968}