[Chromecast] Use embedder_support API for chromium user-agent

As we already completed user-agent reduction in desktop platforms and
Android platform, we plan to remove the feature flag kReduceUserAgent
which used to user-agent reduction origin trial.

This CL removes the feature flags kReduceUserAgent dependency on GetChromiumUserAgent and updates GetChromiumUserAgent to use the reduced user-agent as default instead of create custom user-agent to leak product full version and OsCpu information.

Change-Id: I34f676e0c86b8885434b849f2a14a666c020ce26
Bug: 1258063
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4569003
Commit-Queue: Victor Tan <victortan@chromium.org>
Reviewed-by: Vigen Issahhanjan <vigeni@google.com>
Reviewed-by: Shawn Quereshi <shawnq@google.com>
Cr-Commit-Position: refs/heads/main@{#1151482}

diff --git a/chromecast/common/DEPS b/chromecast/common/DEPS
index bc28542..77e659cb 100644
--- a/chromecast/common/DEPS
+++ b/chromecast/common/DEPS

@@ -1,6 +1,7 @@
 include_rules = [
   "+components/cdm/common",
   "+components/cast/common",
+  "+components/embedder_support/user_agent_utils.h",
   "+components/services/heap_profiling/public/cpp",
   "+components/url_matcher",
   "+components/version_info",