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This CL turns on ThinLTO optimizations for Android by default for all
official builds.

This CL is expected to increase link times of official builds by
something around 2.5x CPU/2x wall time. More build capacity was requested
in http://crbug.com/883563.

This is also a pretty wide-reaching CL. A size decrease of a hundred
KB is expected, and we expect that the CPU-bound parts of Chrome will
generally be faster. If there are slowdowns, they should be very few in
number, and very small.

ThinLTO is a type of link-time optimization, which defers actual
code-generation and some optimization until link-time. This is very
beneficial to the optimizer, since it now has visibility into
functions/constants that, at a per-TU level, would be opaque. ThinLTO is
already deployed on Linux and Android, but our default Linux configuration
does *not* use optimizations. This CL specifically enables these
optimizations for Android.

      size/build time measurements.

Bug: 807147
Test: Perf testing on nexus-5 (crbug.com/836550); manual binary
Change-Id: I7e07d9d9cb1dadd077adf44ca6217c40e13b00a8
Reviewed-on: https://chromium-review.googlesource.com/c/1149061Reviewed-by: agrieve <agrieve@chromium.org>
Reviewed-by: Peter Collingbourne <pcc@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613198}