Simplify the thunk targets since we don't support apps as components

Simplify the thunk targets since we don't support apps as components

Since we don't support using the component build to produce mojo apps,
we can simplify the build targets in a few ways:

*) every mojo_native_application must depend on the c system thunks,
so just make that part of the template instead of requiring the dep
*) there's no such thing as depending on gles2 headers from a component,
so delete the forwarding group.

Most targets that want to use the gles2 headers in a mojo context
want to depend on an implementation through the thunks, so
//mojo/public/c/gles2 does just that. A smaller number of targets (such
as the implementation of the thunks) want to just depend on the headers
but not an impl, so they can depend on //mojo/public/c/gles2:headers.
The //mojo/public/gles target isn't that useful since the only thing we
expose is a set of C entry points.

We can probably also simplify the c system targets, but that's trickier
due to more extensive use from the chromium side.

BUG=438701
R=viettrungluu@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/c41ad161b87d3631bf3a44170cecb5d8634d681c

[viettrungluu, 2014-12-04 00:23:20]

LGTM. You break it, you buy it.

[jamesr, 2014-12-04 00:25:14]

Committed patchset #1 (id:1) manually as
c41ad161b87d3631bf3a44170cecb5d8634d681c (presubmit successful).