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Previously, the code would create a set from all the known
flags and compare the specified flags against them.

But actually this is less efficient than just the O(n*m) search,
because n being the number of flags set by the user and m being
the number of available flags, since n will likely be small or 0
and then we don't need to do a lot of work. In particular, this
also avoids copying a bunch of strings.

Bug: 904575
Change-Id: I63e59d439c432338aea71eb6258e8f8b42f04921
Reviewed-on: https://chromium-review.googlesource.com/c/1334413
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: ssid <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608128}