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Normally when grit actions are defined during `gn gen`, we don't
include all of the files that the .grd file depends on as inputs,
because we don't need to; grit generates a .d file for dependencies
so that rebuilds will happen correctly. And, computing the 
dependencies is slow (it's equivalent to calling grit).

However, if we don't include the dependencies in the gn build graph,
then `analyze` doesn't know about them, and hence a change to a file
that grit dependencies on will be ignored.

This CL changes things such that when `gn gen` is being run as part
of the `analyze` step, we do set the extra flag to compute the
dependencies. This'll slow GN down, but that's better than 
misreporting dependencies.

In some cases, the .grd files try to include generated files
(which may not exist at GN time). To work around this, you can
mark the grit targets as `source_is_generated = true`; the
downside of that is that that target will be skipped just as if
compute_grit_inputs_for_analyze=False, however, this is still an
improvement over not checking *any* of the grit targets. A better
fix would be to have grit_info ignore any paths in the generated
directory if they don't exist, and I'll try to do that in a 
follow-up patch.

R=brettw@chromium.org
BUG=639328

Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I9f6702b06f8a41a159e4599bf192db0abf9e77eb
Reviewed-on: https://chromium-review.googlesource.com/545150
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487135}