media: enable VEA |min_resolution| for PeerConnection on CrOS

crrev.com/c/1828078 added and wired the video encode acceleration
minimum resolution; this info is used from peerconnection's
rtc_video_encoder.cc and video_codec_factory.cc to avoid using a hw
accelerator for resolutions below the given one, if the appropriate
flag is enabled.

This CL enables the use of such mode by default on ChromeOS.

TEST=https://codepen.io/miguelao/full/jONQBYb on kohaku, then
inspecting chrome:webrtc-internals, verifying that resolutions
below 321x241 are encoded using libvpx while larger ones are
encoded using "VaapiVidoeEncodeAccelerator", i.e. the video encode
accelerator in Chrome, in this case VA-API. Joining a
Meet conference with a few bots and all working as expected,
see https://imgur.com/a/B1713ID

Bug: 1052352
Change-Id: I7def6906955bdce182a594f6092d462a25bd37a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2057244Reviewed-by: Florent Castelli <orphis@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742159}

third_party/blink/common/features.cc

@@ -339,8 +339,15 @@ const base::FeatureParam<int> kForceDarkBackgroundLightnessThresholdParam{
     &kForceWebContentsDarkMode, "background_lightness_threshold", -1};
 
 // Instructs WebRTC to honor the Min/Max Video Encode Accelerator dimensions.
-const base::Feature kWebRtcUseMinMaxVEADimensions{
-    "WebRtcUseMinMaxVEADimensions", base::FEATURE_DISABLED_BY_DEFAULT};
+const base::Feature kWebRtcUseMinMaxVEADimensions {
+  "WebRtcUseMinMaxVEADimensions",
+  // TODO(crbug.com/1008491): enable other platforms.
+#if defined(OS_CHROMEOS)
+      base::FEATURE_ENABLED_BY_DEFAULT
+#else
+      base::FEATURE_DISABLED_BY_DEFAULT
+#endif
+};
 
 // Blink garbage collection.
 // Enables compaction of backing stores on Blink's heap.