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The previous code removed symbols from clang builds based on what GYP used to
do. But GYP changed this logic when clang was set to the default. GN was then
left with clang on by default, but no symbols.

This adds some additional GYP flag parity work.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/501173002

Cr-Commit-Position: refs/heads/master@{#291803}