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ProtoZero plugin is a custom C++ stubs generator for protobuf compiler.
These stubs are Tracing V2 specific and meet perfomance and memory
requirements being less functional than conventional ones.
This CL has no complete implementation (see crrev.com/2083373002) but
contains all invasive changes to build files.
See crrev.com/2082693002 for background about protobuf plugin support.

BUG=608721

Review-Url: https://codereview.chromium.org/2147613002
Cr-Commit-Position: refs/heads/master@{#405281}