Convert media constants to constexpr.

media/base/seekable_buffer_unittest.cc

Index: media/base/seekable_buffer_unittest.cc
diff --git a/media/base/seekable_buffer_unittest.cc b/media/base/seekable_buffer_unittest.cc
index 8d135590788fbbeaea99375ad49e5892ae2288d3..f8178c3430c8db4d194dd8184f82ccd9c5f26399 100644
--- a/media/base/seekable_buffer_unittest.cc
+++ b/media/base/seekable_buffer_unittest.cc
@@ -296,7 +296,7 @@ TEST_F(SeekableBufferTest, AllMethods) {
 }
 
 TEST_F(SeekableBufferTest, GetTime) {
-  const int64_t kNoTS = kNoTimestamp().ToInternalValue();
+  const int64_t kNoTS = kNoTimestamp.ToInternalValue();
   const struct {
     int64_t first_time_useconds;
     int64_t duration_useconds;
@@ -332,8 +332,8 @@ TEST_F(SeekableBufferTest, GetTime) {
     { 5, 8000000, kWriteSize, 8000005 },
   };
 
-  // current_time() must initially return kNoTimestamp().
-  EXPECT_EQ(kNoTimestamp().ToInternalValue(),
+  // current_time() must initially return kNoTimestamp.
+  EXPECT_EQ(kNoTimestamp.ToInternalValue(),
             buffer_.current_time().ToInternalValue());
 
   scoped_refptr<DataBuffer> buffer = DataBuffer::CopyFrom(data_, kWriteSize);