media/gpu/test: Fix video decoder tests reading out-of-bounds data from video.

This CL changes the video_decode_accelerator_(perf_)tests to not read data
beyond the end of the test video file, by adding proper bounds checking.

TEST=./video_decode_accelerator_tests \
  vda_sanity-vp90_2_17_show_existing_frame_20190109.vp9

BUG=None

Change-Id: I486eb6eb89fd13d9985ee518178b65dadf158e64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775944Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693681}

media/gpu/test/video_decode_accelerator_unittest_helpers.cc

@@ -157,6 +157,12 @@ std::string EncodedDataHelper::GetBytesForNextFrame() {
 
   uint32_t frame_size = *reinterpret_cast<uint32_t*>(&data_[pos]);
   pos += 12;  // Skip frame header.
+  // Make sure we are not reading out of bounds.
+  if (pos + frame_size > data_.size()) {
+    LOG(ERROR) << "Unexpected data encountered while parsing frame";
+    next_pos_to_decode_ = data_.size();
+    return bytes;
+  }
   bytes.append(data_.substr(pos, frame_size));
 
   // Update next_pos_to_decode_.