Enable PreferConstantFramerate for power test

BUG=b:173761858, b:174521673
TEST=power_VideoCall passes

Change-Id: I8595d96a1f3767d0ffa38485b7c5b75df554ee39
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/2599679
Tested-by: Wei Lee <wtlee@chromium.org>
Auto-Submit: Wei Lee <wtlee@chromium.org>
Reviewed-by: Puthikorn Voravootivat <puthik@chromium.org>
Commit-Queue: Wei Lee <wtlee@chromium.org>
diff --git a/client/cros/power/power_test.py b/client/cros/power/power_test.py
index 6f8276e..de89c69 100644
--- a/client/cros/power/power_test.py
+++ b/client/cros/power/power_test.py
@@ -95,10 +95,12 @@
     def get_extra_browser_args_for_camera_test(self):
         """Return Chrome args for camera power test."""
         ret = [
-            # No pop up to ask permission to record video.
-            '--use-fake-ui-for-media-stream',
-            # Allow 2 windows side by side.
-            '--force-tablet-mode=clamshell',
+                # No pop up to ask permission to record video.
+                '--use-fake-ui-for-media-stream',
+                # Allow 2 windows side by side.
+                '--force-tablet-mode=clamshell',
+                # Prefer using constant frame rate for camera streaming.
+                '--enable-features=PreferConstantFrameRate',
         ]
 
         # Use fake camera for DUT without camera, e.g. chromebox.