Stop checking VP8BaseHeavyTl3RateAllocation field trial on every frame.

- Centralize field trial string reading to RateControlSettings
- Cache RateControlSettings at all production code use sites

Bug: None
Change-Id: I0dbce9cc97fea0bc780982e7ef270b417a8c15bf
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158664
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29680}

modules/video_coding/media_opt_util.h

@@ -17,6 +17,7 @@
 #include <memory>
 
 #include "modules/video_coding/internal_defines.h"
+#include "rtc_base/experiments/rate_control_settings.h"
 #include "rtc_base/numerics/exp_filter.h"
 
 namespace webrtc {
@@ -183,6 +184,8 @@ class VCMFecMethod : public VCMProtectionMethod {
   enum { kMaxBytesPerFrameForFecLow = 400 };
   // Max bytes/frame for frame size larger than VGA, ~200k at 25fps.
   enum { kMaxBytesPerFrameForFecHigh = 1000 };
+
+  const RateControlSettings rate_control_settings_;
 };
 
 class VCMNackFecMethod : public VCMFecMethod {