Revert of Introduce an IsMutable method on VideoFrameBuffer.

webrtc/common_video/i420_buffer_pool_unittest.cc

Index: webrtc/common_video/i420_buffer_pool_unittest.cc
diff --git a/webrtc/common_video/i420_buffer_pool_unittest.cc b/webrtc/common_video/i420_buffer_pool_unittest.cc
index 273b72dc915859c00ec63ed1d73e781ea0b18f22..b030ee774aa8135cdc63c6f299292ca45f343e8f 100644
--- a/webrtc/common_video/i420_buffer_pool_unittest.cc
+++ b/webrtc/common_video/i420_buffer_pool_unittest.cc
@@ -55,7 +55,7 @@
   // Check that created buffers are exclusive so that they can be written to.
   I420BufferPool pool;
   rtc::scoped_refptr<VideoFrameBuffer> buffer = pool.CreateBuffer(16, 16);
-  EXPECT_TRUE(buffer->IsMutable());
+  EXPECT_TRUE(buffer->HasOneRef());
 }
 
 TEST(TestI420BufferPool, FrameValidAfterPoolDestruction) {
@@ -64,7 +64,7 @@
     I420BufferPool pool;
     buffer = pool.CreateBuffer(16, 16);
   }
-  EXPECT_TRUE(buffer->IsMutable());
+  EXPECT_TRUE(buffer->HasOneRef());
   EXPECT_EQ(16, buffer->width());
   EXPECT_EQ(16, buffer->height());
   // Try to trigger use-after-free errors by writing to y-plane.