Check for GN args using the union of all rather than individually.

Check for GN args using the union of all rather than individually.

Previously we would check for unused args on a per-build basis. In the GYP build we run the build multiple times, and not all args might be declared in all types of builds. For example, clang args would only be relevant for clang builds, which might only be in the host GYP build.

Makes unused args errors nonfatal. This was coming up a lot in benign cases. It's important to notify people about unused arguments, but it's not critical to block progress on this problem.

R=scottmg@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=251080

[scottmg, 2014-02-11 21:53:51]

lgtm

[brettw, 2014-02-13 18:03:48]

Committed patchset #2 manually as r251080 (presubmit successful).