Use container::back() and container::pop_back().

chromecast/media/cma/pipeline/audio_video_pipeline_impl_unittest.cc

Index: chromecast/media/cma/pipeline/audio_video_pipeline_impl_unittest.cc
diff --git a/chromecast/media/cma/pipeline/audio_video_pipeline_impl_unittest.cc b/chromecast/media/cma/pipeline/audio_video_pipeline_impl_unittest.cc
index 2e186c9786fd25fdd43343d6719d7133d29dd310..074714eef137198cce70529818223580d1458b7c 100644
--- a/chromecast/media/cma/pipeline/audio_video_pipeline_impl_unittest.cc
+++ b/chromecast/media/cma/pipeline/audio_video_pipeline_impl_unittest.cc
@@ -158,7 +158,7 @@ class PipelineHelper {
       frame_specs[k].size = kFrameSize;
       frame_specs[k].has_decrypt_config = encrypted_;
     }
-    frame_specs[frame_specs.size() - 1].is_eos = true;
+    frame_specs.back().is_eos = true;
 
     std::unique_ptr<FrameGeneratorForTest> frame_generator(
         new FrameGeneratorForTest(frame_specs));