[Chromecast] Add buildflag to enable mobile user agent string

Now that we display a wider range of web content, some sites are
serving desktop sites with images too large for our memory limits
and so on.

Longer-term, it may be possible to use this UA string for all products.

Bug: internal b/112465626
Change-Id: I5c30b1966f75143eb5d99efabfb9093bf6159e90
Reviewed-on: https://chromium-review.googlesource.com/1175087Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Commit-Queue: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583070}

chromecast/BUILD.gn

@@ -583,6 +583,7 @@ buildflag_header("chromecast_buildflags") {
     "ENABLE_CHROMECAST_EXTENSIONS=$enable_chromecast_extensions",
     "ENABLE_CAST_FRAGMENT=$enable_cast_fragment",
     "IS_ANDROID_THINGS_NON_PUBLIC=$is_android_things_non_public",
+    "USE_ANDROID_USER_AGENT=$use_android_user_agent",
     "USE_CHROMECAST_CDMS=$use_chromecast_cdms",
   ]
 }

chromecast/chromecast.gni

@@ -83,6 +83,11 @@ declare_args() {
 
   # unified flag to pick monotonic_clock OR monotonic_clock_raw
   media_clock_monotonic_raw = !is_android && !is_cast_desktop_build
+
+  # Include 'Android' in user agent string to avoid being served desktop
+  # versions of websites.
+  # TODO(halliwell): consider making this default for all Cast hardware.
+  use_android_user_agent = false
 }
 
 declare_args() {

chromecast/common/cast_content_client.cc

@@ -72,6 +72,8 @@ std::string GetUserAgent() {
 #if defined(OS_ANDROID)
       "Linux; ",
       BuildAndroidOsInfo().c_str()
+#elif BUILDFLAG(USE_ANDROID_USER_AGENT)
+                      "Linux; ", "Android"
 #else
       "X11; ",
       content::BuildOSCpuInfo(false /* include_android_build_number */).c_str()