chromecast: media: Add missing monotonic_clock dependency

This fixes the following error from gn check:

The target:
//chromecast/media:cast_media_unittests
is including a file from the target:
//chromecast/media/base:monotonic_clock

It's usually best to depend directly on the destination target.
In some cases, the destination target is considered a subcomponent
of an intermediate target. In this case, the intermediate target
should depend publicly on the destination to forward the ability
to include headers.

Dependency chain (there may also be others):
//chromecast/media:cast_media_unittests -->
//chromecast/media:media -->
//chromecast/media/audio:audio --[private]-->
//chromecast/media/base:monotonic_clock

Bug: b/140414631

Change-Id: Ief92c8a19020b7a345d6b013207511a35b49fcb2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1784003
Auto-Submit: Michael Spang <spang@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692998}

chromecast/media/BUILD.gn

@@ -64,6 +64,7 @@ test("cast_media_unittests") {
     "//chromecast/base/metrics:test_support",
     "//chromecast/common/mojom",
     "//chromecast/media/audio/capture_service:unittests",
+    "//chromecast/media/base:monotonic_clock",
     "//chromecast/media/cma:test_support",
     "//chromecast/media/cma:unittests",
     "//chromecast/public",