[Chromecast] Fix GPU process crash in cast_shell_browsertests

The GPU process crash when using headless ozone platform, as
the GPU process tries to access OverlayManager that is not
initialized. Disable overlay for this unittest by setting an
empty --enable-hardware-overlays flag for overlay strategies.

Bug: b/145926049
Test: cast_shell_browsertests
Change-Id: Ia5737bb227023cb9163a688da8553ce24eeedc85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2003210Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Jiaqi Han <jiaqih@google.com>
Cr-Commit-Position: refs/heads/master@{#732121}

chromecast/BUILD.gn

@@ -147,6 +147,7 @@ cast_test_group("cast_tests") {
         "--no-sandbox",
         "--enable-local-file-accesses",
         "--test-launcher-jobs=1",
+        "--enable-hardware-overlays=\"\"",
       ]
 
       gtest_excludes = []