Make official Blimp Engine builds have a symbol level of 0
Make official Blimp Engine builds have a symbol level of 0

Using the 'is_official_build' gn flag pulls in a bunch of debugging
symbols that we don't need for the engine, and in fact will bloat the
binary size. Using a symbol level of 0 has no effect on the breakpad
symbol file generated by dump_app_syms.py, so we don't lose any of our
desired debugging capabilities.

Tested by building with the buildbot configuration, uploading symbols,
and causing an engine crash with 'kill -6'.

BUG=613186

Review-Url: https://codereview.chromium.org/1987313004
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#394906}

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