Add function for getting supported H264 level from max resolution and fps

webrtc/common_video/h264/profile_level_id.h

Index: webrtc/common_video/h264/profile_level_id.h
diff --git a/webrtc/common_video/h264/profile_level_id.h b/webrtc/common_video/h264/profile_level_id.h
index c7aeff9f0143e6657883af99ecbf9486f447d84b..bf8ba85a67cdd00fc44a7d1f5e2e996f7b6ac9e0 100644
--- a/webrtc/common_video/h264/profile_level_id.h
+++ b/webrtc/common_video/h264/profile_level_id.h
@@ -61,6 +61,11 @@ struct ProfileLevelId {
 // profile level id.
 rtc::Optional<ProfileLevelId> ParseProfileLevelId(const char* str);
 
+// Given the maximum supported frame size (in pixels) and fps, return the
+// highest H264 level the encoder/decoder is guaranteed to support, or
+// return no level if not even level 1 is supported.

[hta-webrtc, 2016/11/03 14:04:32]

This can be made clearer.
Something like:

Given that a decoder supports up to a given frame size (in pixels) at up to a
given number of frames per second, return the highest H.264 level where it can
guarantee that it will be able to support all valid encoded streams that are
within that level.

For an encoder, I think the logic will be different - you would want the level
that would guarantee that if the other end agreed to handle it, you could drive
the encoder to maximum performance without overrunning the decoder.

Thus, if 1280*720@30 is slightly more than Level 3.1 but less than level 4.1:

SupportedDecoderLevel(1280*720, 30) = Level3.1
SupportedEncoderLevel(1280*720, 30) = Level4.1

[magjed_webrtc, 2016/11/03 14:53:39]

You are right about this and I thought about adding two versions when I
implemented this named LowerBoundLevel/UpperBoundLevel or
SupportedLevel/RequiredLevel. However, I don't think the SupportedEncoderLevel
variant will ever be needed since we should base our offered level solely on
decode level. We offer our decode level in SDP, and we limit the input to the
encoder based on remote decode level.

[magjed_webrtc, 2016/11/03 14:54:52]

I updated the comment according to your suggestion though.

+rtc::Optional<Level> SupportedLevel(int max_frame_pixel_count, int max_fps);
+
 // Returns canonical string representation as three hex bytes of the profile
 // level id, or returns empty string for invalid profile level id.
 std::string ProfileLevelIdToString(const ProfileLevelId& profile_level_id);