Remove MutableStyle usage from blink_unittests.

There should be no behaviour change as a result of this patch.

Bug: 813068
Change-Id: I627eeea190435b5136f5382c2d28bab66f9eb617
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1639117Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665235}

third_party/blink/renderer/core/animation/keyframe_effect_test.cc

@@ -451,20 +451,19 @@ TEST_F(KeyframeEffectTest, CheckCanStartAnimationOnCompositorBadTarget) {
       MakeGarbageCollected<KeyframeEffect>(element, effect_model, timing);
 
   // If the target has a CSS offset we can't composite it.
+  element->SetInlineStyleProperty(CSSPropertyID::kOffsetPosition, "50px 50px");
   UpdateAllLifecyclePhasesForTest();
-  element->MutableComputedStyle()->SetOffsetPosition(
-      LengthPoint(Length::Percent(50.0), Length::Auto()));
+
   ASSERT_TRUE(element->GetComputedStyle()->HasOffset());
   EXPECT_TRUE(keyframe_effect->CheckCanStartAnimationOnCompositor(
                   nullptr, animation_playback_rate) &
               CompositorAnimations::kTargetHasCSSOffset);
 
   // If the target has multiple transform properties we can't composite it.
+  element->SetInlineStyleProperty(CSSPropertyID::kRotate, "90deg");
+  element->SetInlineStyleProperty(CSSPropertyID::kScale, "2 1");
   UpdateAllLifecyclePhasesForTest();
-  element->MutableComputedStyle()->SetRotate(
-      RotateTransformOperation::Create(100, TransformOperation::kRotateX));
-  element->MutableComputedStyle()->SetScale(
-      ScaleTransformOperation::Create(2, 1, TransformOperation::kScaleX));
+
   EXPECT_TRUE(keyframe_effect->CheckCanStartAnimationOnCompositor(
                   nullptr, animation_playback_rate) &
               CompositorAnimations::kTargetHasMultipleTransformProperties);

third_party/blink/renderer/core/paint/image_element_timing_test.cc

@@ -128,10 +128,8 @@ TEST_F(ImageElementTimingTest, BackgroundImageRemoved) {
     <div id="target">/div>
   )HTML");
   LayoutObject* object = GetLayoutObjectByElementId("target");
-  ImageResourceContent* content = object->MutableStyle()
-                                      ->AccessBackgroundLayers()
-                                      .GetImage()
-                                      ->CachedImage();
+  ImageResourceContent* content =
+      object->Style()->BackgroundLayers().GetImage()->CachedImage();
   UpdateAllLifecyclePhasesForTest();
   EXPECT_EQ(GetBackgroundImagesNotified().size(), 1u);
   EXPECT_TRUE(