[Overuse] Implement Resource and ResourceUsageListener.

The Resource interface (previously a skeleton not used outside of
testing) is updated to inform listeners of changes to resource
usage. Debugging methods are removed (Name, UsageUnitsOfMeasurements,
CurrentUsage). The interface is implemented by
OveruseFrameDetectorResourceAdaptationModule's inner classes
EncodeUsageResource and QualityScalerResource.

The new ResourceUsageListener interface is implemented by
OveruseFrameDetectorResourceAdaptationModule. In order to avoid adding
AdaptationObserverInterface::AdaptReason to the ResourceUsageListener
interface, the module figures out if the reason is "kCpu" or "kQuality"
by looking which Resource object triggered
OnResourceUsageStateMeasured(). These resources no longer need an
explicit reference to OveruseFrameDetectorResourceAdaptationModule and
could potentially be used by a different module.

In this CL, AdaptationObserverInterface::AdaptDown()'s return value is
still needed by QualityScaler. This is mirrored in the return value of
ResourceUsageListener::OnResourceUsageStateMeasured(). A TODO is added
to remove it and a comment explains how the current implementation
seems to break the contract of the method (as was the case prior to
this CL).

Follow-up work include:
- Move EncodeUsageResource and QualityScalerResource to separate files.
- Make resources injectable, allowing fake resources in testing and
  removing OnResourceOveruseForTesting() methods.
  (Investigate adding the necessary input signals to the Resource
  interface or relevant sub-interfaces so that the module does not need
  to know which Resource implementation is used.)
- And more! See whiteboard :)

Bug: webrtc:11222
Change-Id: I0a46ace4a2e617874e3ee97e67e3a199fef420a2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168180
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Evan Shrubsole <eshr@google.com>
Cr-Commit-Position: refs/heads/master@{#30469}

modules/video_coding/utility/quality_scaler.h

@@ -28,6 +28,11 @@ namespace webrtc {
 
 // An interface for signaling requests to limit or increase the resolution or
 // framerate of the captured video stream.
+// TODO(hbos): Can we remove AdaptationObserverInterface in favor of
+// ResourceUsageListener? If we need to adapt that is because of resource usage.
+// A multi-stream and multi-resource aware solution needs to sparate the notion
+// of being resource constrained from the decision to downgrade a specific
+// stream.
 class AdaptationObserverInterface {
  public:
   // Indicates if the adaptation is due to overuse of the CPU resources, or if