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This change removes the current execution profile _mojo_ representation
in favor of directly writing the data in the protocol buffer format used
for UMA upload. The reasons for using this format are:
1. it's the format that we ultimately need the data in;
2. it's the most memory-efficient representation available to us, because
   of the protocol buffer varint encoding;
3. the serialized protocol buffers are trivially compressible.

We generate execution profiles in non-browser processes, so this means
writing the protocol buffer representation in those processes and sending
over mojo to the browser process, where the UMA upload takes place.

We're making this change as part of a refactoring to expand the profiling
scenarios supported by the UMA Sampling Profiler. A key aspect of this
refactoring is reducing memory consumption of the execution profile
representation. See http://crbug.com/850148 for examples of memory usage
problems with this representation.

Bug: 851163, 804942
Change-Id: Ied88d6fd932587707c1f5a4c79acd77d5911f6ee
Reviewed-on: https://chromium-review.googlesource.com/1157547
Commit-Queue: Mike Wittman <wittman@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582209}