media: Increase DecoderBuffer::kPaddingSize to 64

AV_INPUT_BUFFER_PADDING_SIZE has been increased to 64 in FFmpeg:
FFmpeg/FFmpeg@6e80079

BUG=777484

Change-Id: I1bd68d1c1b0c3131f28d6e07e1444b89800c09db
Reviewed-on: https://chromium-review.googlesource.com/889686
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532593}

media/base/decoder_buffer.h

@@ -37,7 +37,7 @@ class MEDIA_EXPORT DecoderBuffer
     : public base::RefCountedThreadSafe<DecoderBuffer> {
  public:
   enum {
-    kPaddingSize = 32,
+    kPaddingSize = 64,
 #if defined(ARCH_CPU_ARM_FAMILY)
     kAlignmentSize = 16
 #else