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This CL changes the extensions build to have BUILD.gn's in (almost) all
leaf directories. This accomplishes several things,

a.) Makes it very clear of what files belong to which component/API. For
example, to see which files exist in the audio API, one needs to parse
the massive single //extensions/browsers/BUILD.gn to find the main API
source list, then look if any of the exception conditions (is_chromeos,
is_linux, etc) also contain any of the audio API files. This is now all
in one //extensions/browser/api/audio/BUILD.gn, making it easier to parse
and understand the build of each component.

b.) It makes dependencies clearer. Currently we have no idea which
dependency in the huge dependency list in //extensions/browser/BUILD.gn is
needed by which API. With individual build files, we can now specify
dependencies for just the component that needs them. This CL tries to
do this but some dependencies are used by enough components that we've just
included them for all (//content/browser/public, //skia). This is not ideal
but at worst, it is still much better than the current build.

Since we're now moving APIs over from //chrome/browser/extensions over to
//extensions/browser, this becomes even more important. All APIs that get
moved over now can use this new format for their build files.

jamescook@, rockot@, xiyuan@ - FYI

R=rdevlin.cronin@chromium.org
BUG=641155

Review-Url: https://codereview.chromium.org/2336843002
Cr-Commit-Position: refs/heads/master@{#418729}