This moves the work of finding headers from `gn gen` time into the action itself. We can do this safely now because we're constructing a skia.h.d deps file, which Ninja uses to track if-these-are-dirty-then-this-is-dirty relationships. Everything can now live in one handy find_headers.py.
Upshot is, `gn gen` runs ~50ms faster, and I think the code's clearer this way too.
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4065
Change-Id: I2f1e7adf68be8e961eb77596786a795134bbb9a6
Reviewed-on: https://skia-review.googlesource.com/4065
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
