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This adds a new GN template for generating static extensions API
library bundles from sets of schemas. It also adds invocations of
the template for core extensions and apps API targets.

BUG=None
TBR=kalman

Review URL: https://codereview.chromium.org/256453002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266341 0039d316-1c4b-4281-b951-d872f2087c98