Attempt to fix AudioDecoderSelectorTest and VideoDecoderSelectorTest failures on MSan.

https://chromium-review.googlesource.com/c/chromium/src/+/952207 removed
the initialization of 4 values from constructors, but some tests still
read these values.  Set them to default values.

See comments on the original review for links to failing bots.

TBR=dalecurtis@chromium.org

Bug: 801245
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I346b16102492e2683e7a7cd4d626eeb4c25f1e12
Reviewed-on: https://chromium-review.googlesource.com/958208
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542400}

media/filters/audio_decoder_selector_unittest.cc

@@ -224,8 +224,8 @@ class AudioDecoderSelectorTest : public ::testing::Test {
 
   std::unique_ptr<AudioDecoderSelector> decoder_selector_;
 
-  StrictMock<MockAudioDecoder>* decoder_1_;
-  StrictMock<MockAudioDecoder>* decoder_2_;
+  StrictMock<MockAudioDecoder>* decoder_1_ = nullptr;
+  StrictMock<MockAudioDecoder>* decoder_2_ = nullptr;
   std::vector<std::unique_ptr<AudioDecoder>> all_decoders_;
   std::unique_ptr<AudioDecoder> selected_decoder_;
 

media/filters/video_decoder_selector_unittest.cc

@@ -217,8 +217,8 @@ class VideoDecoderSelectorTest : public ::testing::Test {
 
   std::unique_ptr<VideoDecoderSelector> decoder_selector_;
 
-  StrictMock<MockVideoDecoder>* decoder_1_;
-  StrictMock<MockVideoDecoder>* decoder_2_;
+  StrictMock<MockVideoDecoder>* decoder_1_ = nullptr;
+  StrictMock<MockVideoDecoder>* decoder_2_ = nullptr;
   std::vector<std::unique_ptr<VideoDecoder>> all_decoders_;
   std::unique_ptr<VideoDecoder> selected_decoder_;