GitLab

This CL adds a metadata to existing TileGroup proto.
And store it in the Tile DB.
To avoid being removed during tile download, this TileGroup
is using a special key and saved separately.
A utility class is introduced to sort the tiles.

Bug: 1096224
Change-Id: I81f4471e5e89794d3538a4b7522c6b22c36e15b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2417088
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808193}