media/gpu/vdatest: remove --rendering_warm_up option

This option was a no-op, and is being removed from autotests.

BUG=819542
TEST=Verified that autotests were still passing
CQ-DEPEND=CL:952036

Change-Id: I072a56faf78a3c453e9195e7a4f8cf5599872796
Reviewed-on: https://chromium-review.googlesource.com/954768Reviewed-by: Wu-Cheng Li <wuchengli@chromium.org>
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543971}

media/gpu/video_decode_accelerator_unittest.cc

@@ -1850,10 +1850,6 @@ int main(int argc, char** argv) {
       LOG_ASSERT(base::StringToDouble(input, &media::g_rendering_fps));
       continue;
     }
-    if (it->first == "rendering_warm_up") {
-      // TODO(owenlin): Remove this after autotest stop using it.
-      continue;
-    }
     // TODO(owenlin): Remove this flag once it is not used in autotest.
     if (it->first == "disable_rendering") {
       media::g_rendering_fps = 0;