Android: Add docs/speed/binary_size/metrics.md

Change-Id: I7b96f53f5d3385bb192224078a56bbe36e92eb07
Reviewed-on: https://chromium-review.googlesource.com/680054
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504483}
3 files changed