Add field trial entry for CSSReducedFontLoadingInvalidations

The feature has been tested in Beta for a while but doesn't have a field
trial entry.

This patch adds the entry for sanity, and as a preparation to roll out
testing in Stable.

Bug: 441925, 1088373
Change-Id: Id7b903ea83783fcee6158fafd142feb46341389c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209539Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774863}

testing/variations/fieldtrial_testing_config.json

@@ -1203,6 +1203,27 @@
             ]
         }
     ],
+    "CSSReducedFontLoadingInvalidations": [
+        {
+            "platforms": [
+                "android",
+                "android_weblayer",
+                "android_webview",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "CSSReducedFontLoadingInvalidations"
+                    ]
+                }
+            ]
+        }
+    ],
     "CacheStorageEagerReading": [
         {
             "platforms": [

third_party/blink/renderer/core/layout/layout_object.cc

@@ -2038,6 +2038,14 @@ StyleDifference LayoutObject::AdjustStyleDifference(
       diff.SetNeedsPaintInvalidation();
   }
 
+  // TODO(1088373): Pixel_WebGLHighToLowPower fails without this. This isn't the
+  // right way to ensure GPU switching. Investigate and do it in the right way.
+  if (RuntimeEnabledFeatures::CSSReducedFontLoadingInvalidationsEnabled() &&
+      !diff.NeedsPaintInvalidation() && IsLayoutView() && Style() &&
+      !Style()->GetFont().IsFallbackValid()) {
+    diff.SetNeedsPaintInvalidation();
+  }
+
   // The answer to layerTypeRequired() for plugins, iframes, and canvas can
   // change without the actual style changing, since it depends on whether we
   // decide to composite these elements. When the/ layer status of one of these

third_party/blink/renderer/platform/fonts/font_fallback_list.cc

@@ -97,7 +97,10 @@ bool FontFallbackList::LoadingCustomFonts() const {
 }
 
 bool FontFallbackList::ShouldSkipDrawing() const {
-  DCHECK(IsValid());
+  // The DCHECK hit will be fixed by the runtime enabled feature below, so we
+  // don't fix it in the legacy code paths.
+  DCHECK(IsValid() || !RuntimeEnabledFeatures::
+                          CSSReducedFontLoadingLayoutInvalidationsEnabled());
 
   if (!has_loading_fallback_)
     return false;