Make sure --force-prefers-reduced-motion always overrides system setting

After https://crrev.com/e90092dea, the --force-prefers-reduced-motion
flag could be accidentally ignored if the system-level setting was
changed after the browser had started. This CL restores the behavior so
that the flag always wins, and adds a test to ensure that.

Bug: None
Change-Id: I46927e3821f1118f38f4896d07755b3eaebe48b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1921553Reviewed-by: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720197}

ui/gfx/animation/animation.cc

@@ -140,14 +140,15 @@ void Animation::UpdatePrefersReducedMotion() {
 
 // static
 bool Animation::PrefersReducedMotion() {
-  if (!prefers_reduced_motion_.has_value()) {
-    if (base::CommandLine::ForCurrentProcess()->HasSwitch(
-            switches::kForcePrefersReducedMotion)) {
-      prefers_reduced_motion_ = true;
-    } else {
-      UpdatePrefersReducedMotion();
-    }
+  // --force-prefers-reduced-motion must always override
+  // |prefers_reduced_motion_|, so check it first.
+  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kForcePrefersReducedMotion)) {
+    return true;
   }
+
+  if (!prefers_reduced_motion_.has_value())
+    UpdatePrefersReducedMotion();
   return prefers_reduced_motion_.value();
 }
 

ui/gfx/animation/animation.h

@@ -84,6 +84,9 @@ class ANIMATION_EXPORT Animation : public AnimationContainerElement {
   // Should only be called from the browser process, on the UI thread.
   static bool PrefersReducedMotion();
   static void UpdatePrefersReducedMotion();
+  static void SetPrefersReducedMotionForTesting(bool prefers_reduced_motion) {
+    prefers_reduced_motion_ = prefers_reduced_motion;
+  }
 
  protected:
   // Invoked from Start to allow subclasses to prepare for the animation.

ui/gfx/animation/animation_unittest.cc

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include "base/command_line.h"
 #include "base/run_loop.h"
 #include "base/test/task_environment.h"
 #include "build/build_config.h"
@@ -9,6 +10,7 @@
 #include "ui/gfx/animation/animation_delegate.h"
 #include "ui/gfx/animation/linear_animation.h"
 #include "ui/gfx/animation/test_animation_delegate.h"
+#include "ui/gfx/switches.h"
 
 #if defined(OS_WIN)
 #include <windows.h>
@@ -158,4 +160,18 @@ TEST_F(AnimationTest, StartState) {
   EXPECT_EQ(0.0, animation.GetCurrentValue());
 }
 
+///////////////////////////////////////////////////////////////////////////////
+// PrefersReducedMotion tests
+
+TEST_F(AnimationTest, PrefersReducedMotionRespectsOverrideFlag) {
+  base::CommandLine::ForCurrentProcess()->AppendSwitchASCII(
+      switches::kForcePrefersReducedMotion, "1");
+  EXPECT_TRUE(Animation::PrefersReducedMotion());
+
+  // It doesn't matter what the system setting says; the flag should continue to
+  // override it.
+  Animation::SetPrefersReducedMotionForTesting(false);
+  EXPECT_TRUE(Animation::PrefersReducedMotion());
+}
+
 }  // namespace gfx