[cc] Ship smaller interest area on Android

On Android, setting a smaller interest area improves memory, without
regressing latency metrics (with even a very small improvement to
INP). Memory savings are 10-20MiB depending on screen resolution, when
tiles are present.

On desktop though, memory savings are small, and likely not worth
potential regressions. Do not enable there.

Bug: 1459256
Change-Id: I9a5c132f588e38fdf38776413323f87405fbded7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4931597
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1209854}

cc/base/features.cc

@@ -135,9 +135,17 @@ BASE_FEATURE(kReclaimPrepaintTilesWhenIdle,
              "ReclaimPrepaintTilesWhenIdle",
              base::FEATURE_DISABLED_BY_DEFAULT);
 
+// This saves memory on all platforms, but while on Android savings are
+// significant (~10MiB or more of foreground memory), on desktop they were
+// small, so only enable on Android.
 BASE_FEATURE(kSmallerInterestArea,
              "SmallerInterestArea",
-             base::FEATURE_DISABLED_BY_DEFAULT);
+#if BUILDFLAG(IS_ANDROID)
+             base::FEATURE_ENABLED_BY_DEFAULT
+#else
+             base::FEATURE_DISABLED_BY_DEFAULT
+#endif
+);
 
 const base::FeatureParam<int> kInterestAreaSizeInPixels{
     &kSmallerInterestArea, "size_in_pixels", kDefaultInterestAreaSizeInPixels};

cc/base/features.h

@@ -127,6 +127,7 @@ CC_BASE_EXPORT BASE_DECLARE_FEATURE(kReclaimPrepaintTilesWhenIdle);
 CC_BASE_EXPORT BASE_DECLARE_FEATURE(kSmallerInterestArea);
 
 constexpr static int kDefaultInterestAreaSizeInPixels = 3000;
+constexpr static int kDefaultInterestAreaSizeInPixelsWhenEnabled = 500;
 CC_BASE_EXPORT extern const base::FeatureParam<int> kInterestAreaSizeInPixels;
 
 // Whether images marked "no-cache" are cached. When disabled, they are.

third_party/blink/renderer/platform/widget/compositing/layer_tree_settings.cc

@@ -95,8 +95,17 @@ bool IsSmallScreen(const gfx::Size& size) {
 #endif
 
 std::pair<int, int> GetTilingInterestAreaSizes() {
-  int interest_area_size_in_pixels =
-      ::features::kInterestAreaSizeInPixels.Get();
+  int interest_area_size_in_pixels;
+
+  if (base::FeatureList::IsEnabled(::features::kSmallerInterestArea) &&
+      ::features::kInterestAreaSizeInPixels.Get() ==
+          ::features::kInterestAreaSizeInPixels.default_value) {
+    interest_area_size_in_pixels =
+        ::features::kDefaultInterestAreaSizeInPixelsWhenEnabled;
+  } else {
+    interest_area_size_in_pixels = ::features::kInterestAreaSizeInPixels.Get();
+  }
+
   if (interest_area_size_in_pixels ==
       ::features::kInterestAreaSizeInPixels.default_value) {
     return {