Resolve jumbo conflict in video decoders (media/filters)

When building using jumbo, files gets merged. Files and
variables with the same name may merge into the same
namespace or scope and conflict. This happens in the files:
media/filters/aom_video_decoder.cc
media/filters/ffmpeg_video_decoder.cc
media/filters/vpx_video_decoder.cc

This commit solves the issue by renaming the function
GetThreadCount to something more file specific. It also
moves some constants to that function.

Bug: 867350
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Id57aa6cc6ac10acd0073eee032d0d04592ba87e4
Reviewed-on: https://chromium-review.googlesource.com/1151311
Commit-Queue: Chrome Cunningham (In Paris) <chcunningham@chromium.org>
Reviewed-by: Chrome Cunningham (In Paris) <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578593}

media/filters/aom_video_decoder.cc

@@ -27,7 +27,7 @@ extern "C" {
 namespace media {
 
 // Returns the number of threads.
-static int GetThreadCount(const VideoDecoderConfig& config) {
+static int GetAomVideoDecoderThreadCount(const VideoDecoderConfig& config) {
   // Always try to use at least two threads for video decoding. There is little
   // reason not to since current day CPUs tend to be multi-core and we measured
   // performance benefits on older machines such as P4s with hyperthreading.
@@ -187,7 +187,7 @@ void AomVideoDecoder::Initialize(
   aom_codec_dec_cfg_t aom_config = {0};
   aom_config.w = config.coded_size().width();
   aom_config.h = config.coded_size().height();
-  aom_config.threads = GetThreadCount(config);
+  aom_config.threads = GetAomVideoDecoderThreadCount(config);
 
   // TODO(dalecurtis): Refactor the MemoryPool and OffloadTaskRunner out of
   // VpxVideoDecoder so that they can be used here for zero copy decoding off

media/filters/ffmpeg_video_decoder.cc

@@ -31,21 +31,22 @@
 
 namespace media {
 
-// Always use 2 or more threads for video decoding. Most machines today will
-// have 2-8 execution contexts. Using more cores generally doesn't seem to
-// increase power usage and allows us to decode video faster.
-//
-// Handling decoding on separate threads also frees up the pipeline thread to
-// continue processing. Although it'd be nice to have the option of a single
-// decoding thread, FFmpeg treats having one thread the same as having zero
-// threads (i.e., decoding will execute on the calling thread). Yet another
-// reason for having two threads :)
-static const int kDecodeThreads = 2;
-static const int kMaxDecodeThreads = 16;
 
 // Returns the number of threads given the FFmpeg CodecID. Also inspects the
 // command line for a valid --video-threads flag.
-static int GetThreadCount(const VideoDecoderConfig& config) {
+static int GetFFmpegVideoDecoderThreadCount(const VideoDecoderConfig& config) {
+  // Always use 2 or more threads for video decoding. Most machines today will
+  // have 2-8 execution contexts. Using more cores generally doesn't seem to
+  // increase power usage and allows us to decode video faster.
+  //
+  // Handling decoding on separate threads also frees up the pipeline thread to
+  // continue processing. Although it'd be nice to have the option of a single
+  // decoding thread, FFmpeg treats having one thread the same as having zero
+  // threads (i.e., decoding will execute on the calling thread). Yet another
+  // reason for having two threads :)
+  constexpr int kDecodeThreads = 2;
+  constexpr int kMaxDecodeThreads = 16;
+
   // Refer to http://crbug.com/93932 for tsan suppressions on decoding.
   int decode_threads = kDecodeThreads;
 
@@ -414,7 +415,7 @@ bool FFmpegVideoDecoder::ConfigureDecoder(const VideoDecoderConfig& config,
   codec_context_.reset(avcodec_alloc_context3(NULL));
   VideoDecoderConfigToAVCodecContext(config, codec_context_.get());
 
-  codec_context_->thread_count = GetThreadCount(config);
+  codec_context_->thread_count = GetFFmpegVideoDecoderThreadCount(config);
   codec_context_->thread_type =
       FF_THREAD_SLICE | (low_delay ? 0 : FF_THREAD_FRAME);
   codec_context_->opaque = this;

media/filters/vpx_video_decoder.cc

@@ -35,14 +35,14 @@
 
 namespace media {
 
-// Always try to use three threads for video decoding.  There is little reason
-// not to since current day CPUs tend to be multi-core and we measured
-// performance benefits on older machines such as P4s with hyperthreading.
-static const int kDecodeThreads = 2;
-static const int kMaxDecodeThreads = 32;
-
 // Returns the number of threads.
-static int GetThreadCount(const VideoDecoderConfig& config) {
+static int GetVpxVideoDecoderThreadCount(const VideoDecoderConfig& config) {
+  // Always try to use at least two threads for video decoding.  There is little
+  // reason not to since current day CPUs tend to be multi-core and we measured
+  // performance benefits on older machines such as P4s with hyperthreading.
+  constexpr int kDecodeThreads = 2;
+  constexpr int kMaxDecodeThreads = 32;
+
   // Refer to http://crbug.com/93932 for tsan suppressions on decoding.
   int decode_threads = kDecodeThreads;
 
@@ -80,7 +80,7 @@ static std::unique_ptr<vpx_codec_ctx> InitializeVpxContext(
   vpx_codec_dec_cfg_t vpx_config = {0};
   vpx_config.w = config.coded_size().width();
   vpx_config.h = config.coded_size().height();
-  vpx_config.threads = GetThreadCount(config);
+  vpx_config.threads = GetVpxVideoDecoderThreadCount(config);
 
   vpx_codec_err_t status = vpx_codec_dec_init(
       context.get(),