MediaCapabilities: Add "transmission" type.

Add "transmission" type for MediaCapabilities.encodingInfo().
Currently it rejects the promise for "transmission" type.

BUG=817382
TEST=Run blink web tests:
1. autoninja -C out/Debug blink_tests
2. python third_party/blink/tools/run_web_tests.py --debug -t Debug \
   third_party/blink/web_tests/media_capabilities/encodingInfo.html
2. python third_party/blink/tools/run_web_tests.py --debug -t Debug \
   third_party/blink/web_tests/media_capabilities/encodingInfo.html \
   third_party/blink/web_tests/external/wpt/media-capabilities/encodingInfo.html

Change-Id: I372ed4aa5eae4cf2011536034bd543bfc9a36c6f

media-capabilities/encodingInfo.html

@@ -287,7 +287,19 @@
     assert_equals(typeof ability.smooth, "boolean");
     assert_equals(typeof ability.powerEfficient, "boolean");
   });
-}, "Test that encodingInfo returns a valid MediaCapabilitiesInfo objects");
+}, "Test that encodingInfo returns a valid MediaCapabilitiesInfo objects for record type");
+
+promise_test(t => {
+  return navigator.mediaCapabilities.encodingInfo({
+    type: 'transmission',
+    video: minimalVideoConfiguration,
+    audio: minimalAudioConfiguration,
+  }).then(ability => {
+    assert_equals(typeof ability.supported, "boolean");
+    assert_equals(typeof ability.smooth, "boolean");
+    assert_equals(typeof ability.powerEfficient, "boolean");
+  });
+}, "Test that encodingInfo returns a valid MediaCapabilitiesInfo objects for transmission type");
 
 async_test(t => {
   var validTypes = [ 'record', 'transmission' ];