Headless: Add kDisableImageAnimationResync in DeterministicMode

For determinism of animated gifs, set kDisableImageAnimationResync.

Change-Id: Ia609396083534935f0c36d9b7328b50bfc164f8d
Reviewed-on: https://chromium-review.googlesource.com/962122Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543044}

headless/app/headless_shell.cc

@@ -716,6 +716,9 @@ int HeadlessShellMain(int argc, const char** argv) {
     command_line.AppendSwitch(::switches::kRunAllCompositorStagesBeforeDraw);
     command_line.AppendSwitch(::switches::kDisableNewContentRenderingTimeout);
     command_line.AppendSwitch(::switches::kEnableSurfaceSynchronization);
+    // Ensure that image animations don't resync their animation timestamps when
+    // looping back around.
+    command_line.AppendSwitch(::switches::kDisableImageAnimationResync);
 
     // Renderer flags
     command_line.AppendSwitch(cc::switches::kDisableThreadedAnimation);