Fix chromium-style warning for auto in jpeg_decode_accelerator_unittest

This CL fixes the following warnings:

  media/gpu/jpeg_decode_accelerator_unittest.cc:478:8: warning:
  for (const auto& image : g_env->image_data_user_) {
       ^~~~~~~~~~~
       auto* const
  media/gpu/jpeg_decode_accelerator_unittest.cc:486:8: warning:
  for (const auto& image : g_env->image_data_user_) {
       ^~~~~~~~~~~
       auto* const
  2 warnings generated.

BUG=None
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2733013002
Cr-Commit-Position: refs/heads/master@{#454897}
1 file changed