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Currently, startup data is passed from ash-chrome to lacros-chrome
via LacroChromeService::Init() Mojo call. This requires Mojo initialization
so that the data is available only after that.
However, there are some needs that the data would like to be available
earlier.

In order to support that situation, this CL introduces another approach,
where ash-chrome dumps the serialized data into a memory backed file
and lacros-chrome inherits its file descriptor.

For backward compatibility, both lacros-chrome and ash-chrome supports
older protocol, i.e. LacrosChromeService::Init() for now,
while it is now deprecated.
Thus, any {old,new} {lacros-chrome,ash-chrome} combination should work.
It is planned to remove the older protocol support, when newer/older
chromes are distributed enough.

BUG=1155494
TEST=Ran tryjob. Ran {old,new} {lacros-chrome,ash-chrome} combinations
     on DUTs to make sure lacros-chrome can be launched successfully
     via Shelf icon and mojo_connection_lacros_launcher.py script.

Change-Id: I09c7004d07cec9a58ccfc7ee24c0e256149e4951
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2573264
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835744}