Switch DoNotCompositeTrivial3D to be disabled by default

The DoNotCompositeTrivial3D flag has changed many performance tests
(e.g., twitter, see crbug.com/1016343). To reduce the chances of
breaking real sites, I would like to switch to rolling this out via a
(forward) finch experiment. This patch switches the flag to be
disabled-by-default, and I will enable this via finch.

TBR=chrishtr

Bug: 1012775
Change-Id: Ifc9cf670fb95b48995a5072f95eea1e3c570c063
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872659Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Auto-Submit: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707935}

third_party/blink/common/features.cc

@@ -54,7 +54,7 @@ const base::Feature kCSSBackdropFilter{"CSSBackdropFilter",
 
 // When enabled, the compositing of trivial 3D transforms is disabled.
 const base::Feature kDoNotCompositeTrivial3D{"DoNotCompositeTrivial3D",
-                                             base::FEATURE_ENABLED_BY_DEFAULT};
+                                             base::FEATURE_DISABLED_BY_DEFAULT};
 
 // Enable Display Locking JavaScript APIs.
 const base::Feature kDisplayLocking{"DisplayLocking",

third_party/blink/renderer/core/paint/compositing/compositing_reason_finder_test.cc

@@ -38,7 +38,19 @@ TEST_F(CompositingReasonFinderTest, CompositingReasonDependencies) {
                CompositingReason::kComboAllStyleDeterminedReasons);
 }
 
-TEST_F(CompositingReasonFinderTest, DontPromoteTrivial3D) {
+class CompositingReasonFinderTestWithDoNotCompositeTrivial3D
+    : public CompositingReasonFinderTest {
+ public:
+  CompositingReasonFinderTestWithDoNotCompositeTrivial3D() {
+    scoped_feature_list_.InitAndEnableFeature(
+        blink::features::kDoNotCompositeTrivial3D);
+  }
+
+  base::test::ScopedFeatureList scoped_feature_list_;
+};
+
+TEST_F(CompositingReasonFinderTestWithDoNotCompositeTrivial3D,
+       DontPromoteTrivial3D) {
   SetBodyInnerHTML(R"HTML(
     <div id='target'
       style='width: 100px; height: 100px; transform: translateZ(0)'></div>

third_party/blink/renderer/core/paint/compositing/compositing_requirements_updater_test.cc

@@ -3,7 +3,10 @@
 // found in the LICENSE file.
 
 #include "third_party/blink/renderer/core/paint/compositing/compositing_requirements_updater.h"
+
+#include "base/test/scoped_feature_list.h"
 #include "testing/gtest/include/gtest/gtest.h"
+#include "third_party/blink/public/common/features.h"
 #include "third_party/blink/renderer/core/paint/paint_layer.h"
 #include "third_party/blink/renderer/core/paint/paint_layer_scrollable_area.h"
 #include "third_party/blink/renderer/core/testing/core_unit_test_helper.h"
@@ -125,7 +128,19 @@ TEST_F(CompositingRequirementsUpdaterTest,
   EXPECT_EQ(IntRect(0, 0, 100, 100), tracking->Invalidations()[0].rect);
 }
 
-TEST_F(CompositingRequirementsUpdaterTest, NonTrivial3DTransforms) {
+class CompositingRequirementsUpdaterTestWithDoNotCompositeTrivial3D
+    : public CompositingRequirementsUpdaterTest {
+ public:
+  CompositingRequirementsUpdaterTestWithDoNotCompositeTrivial3D() {
+    scoped_feature_list_.InitAndEnableFeature(
+        blink::features::kDoNotCompositeTrivial3D);
+  }
+
+  base::test::ScopedFeatureList scoped_feature_list_;
+};
+
+TEST_F(CompositingRequirementsUpdaterTestWithDoNotCompositeTrivial3D,
+       NonTrivial3DTransforms) {
   ScopedCSSIndependentTransformPropertiesForTest feature_scope(true);
 
   SetBodyInnerHTML(R"HTML(