[Chromecast] Dump the runtime deps for all test targets.

For each foo_unittest binary in cast_test_lists, create a
foo_unittest.runtime_deps file that lists all of the runtime
dependencies (data files, data directories, shared libs, etc.) that a
test needs at runtime.

BUG= internal b/33682037
TEST=gn gen

Review-Url: https://codereview.chromium.org/2629573008
Cr-Commit-Position: refs/heads/master@{#444883}
3 files changed