Add a flag to disable hardware encoding support for Cast Streaming

BUG=404309
TEST=Run this test to see the message for disabling h264 and vp8:
browser_tests --gtest_filter=CastStreaming*
  --disable-cast-streaming-hw-encoding --v=0
  --vmodule=*chrome/renderer/*=3 --log --enable-logging=stderr

Review URL: https://codereview.chromium.org/693683002

Cr-Commit-Position: refs/heads/master@{#302918}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 8801329..565523b8 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1735,6 +1735,13 @@
     kOsDesktop,
     SINGLE_VALUE_TYPE(switches::kOutOfProcessPdf)
   },
+  {
+    "disable-cast-streaming-hw-encoding",
+    IDS_FLAGS_DISABLE_CAST_STREAMING_HW_ENCODING_NAME,
+    IDS_FLAGS_DISABLE_CAST_STREAMING_HW_ENCODING_DESCRIPTION,
+    kOsAll,
+    SINGLE_VALUE_TYPE(switches::kDisableCastStreamingHWEncoding)
+  },
 #if defined(OS_ANDROID)
   {
     "disable-cast",