Adjust the amount of VP8 encoder threads for Android builds.

Current number of threads selection code does not work well
for Android builds - middle and low end devices are having hard time
encoding VGA and QVGA with just one thread.

Increase the amount of vp8 encoder threads for 180p and above resolution.

Also limit maximum number of thread to 3, since for 8 core devices
most of time 4 cores are idle when thermal throttling kicks in.

BUG=b/27946721
R=marpan@webrtc.org

Review URL: https://codereview.webrtc.org/2058753003 .

Cr-Commit-Position: refs/heads/master@{#13142}

webrtc/modules/video_coding/codecs/vp8/vp8_impl.cc

@@ -633,6 +633,20 @@ int VP8EncoderImpl::SetCpuSpeed(int width, int height) {
 }
 
 int VP8EncoderImpl::NumberOfThreads(int width, int height, int cpus) {
+#if defined(ANDROID)
+  if (width * height >= 320 * 180) {
+    if (cpus >= 4) {
+      // 3 threads for CPUs with 4 and more cores since most of times only 4
+      // cores will be active.
+      return 3;
+    } else if (cpus == 3 || cpus == 2) {
+      return 2;
+    } else {
+      return 1;
+    }
+  }
+  return 1;
+#else
   if (width * height >= 1920 * 1080 && cpus > 8) {
     return 8;  // 8 threads for 1080p on high perf machines.
   } else if (width * height > 1280 * 960 && cpus >= 6) {
@@ -645,6 +659,7 @@ int VP8EncoderImpl::NumberOfThreads(int width, int height, int cpus) {
     // 1 thread for VGA or less.
     return 1;
   }
+#endif
 }
 
 int VP8EncoderImpl::InitAndSetControlSettings() {