Fix undefined reference to log2 on android

webrtc/modules/video_coding/utility/quality_scaler.cc

 /*
  *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include "webrtc/modules/video_coding/utility/quality_scaler.h"
 
+#include <math.h>
+
 #include <algorithm>
-#include <cmath>
+
+// TODO(kthelgason): Some versions of Android have issues with log2.
+// See https://code.google.com/p/android/issues/detail?id=212634 for details
+#if defined(WEBRTC_ANDROID)
+#define log2(x) (log(x) / log(2))
+#endif
 
 namespace webrtc {
 
 namespace {
 // Threshold constant used until first downscale (to permit fast rampup).
 static const int kMeasureSecondsFastUpscale = 2;
 static const int kMeasureSecondsUpscale = 5;
 static const int kMeasureSecondsDownscale = 5;
 static const int kFramedropPercentThreshold = 60;
 // Min width/height to downscale to, set to not go below QVGA, but with some
@@ skipping 150 matching lines @@
   scaled_buffer->ScaleFrom(frame);
 
   return scaled_buffer;
 }
 
 void QualityScaler::UpdateTargetResolution(int width, int height) {
   if (width < kMinDownscaleDimension || height < kMinDownscaleDimension) {
     maximum_shift_ = 0;
   } else {
     maximum_shift_ = static_cast<int>(
-        std::log2(std::min(width, height) / kMinDownscaleDimension));
+        log2(std::min(width, height) / kMinDownscaleDimension));
   }
   target_res_ = Resolution{width, height};
 }
 
 void QualityScaler::ClearSamples() {
   framedrop_percent_.Reset();
   average_qp_.Reset();
 }
 
 
 }  // namespace webrtc