Add chromium side flag for enabling/disabling layer squashing.

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
diff --git a/chrome/app/generated_resources.grd b/chrome/app/generated_resources.grd
index 176a503fdebde254f33c73da2e28ae2d43eabff6..0e1a106699ae22d63d7bc3d9a52774d9aece76a4 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -5856,6 +5856,13 @@ Keep your key file in a safe place. You will need it to create new versions of y
       <message name="IDS_FLAGS_FORCE_UNIVERSAL_ACCELERATED_OVERFLOW_SCROLL_MODE_DESCRIPTION" desc="Description of the 'Universal accelerated overflow scroll mode' lab.">
         Puts scrolling content in composited layers, even in those cases where promoting the overflow scrolling element to a stacking context and a containing block would have broken stacking or clipping.
       </message>
+      <message name="IDS_FLAGS_LAYER_SQUASHING_NAME" desc="Name of the 'Layer squashing' lab.">
+        Layer squashing
+      </message>
+      <message name="IDS_FLAGS_LAYER_SQUASHING_DESCRIPTION" desc="Description of the 'layer squashing' lab.">
+        Layers that overlap other composited content need to be composited as well, to ensure correct paint order. This feature enables Blink to paint multiple RenderLayer
+        subtrees into one composited layer so that we may be able to avoid unnecessarily large number of composited layers.
+      </message>
       <message name="IDS_FLAGS_ENABLE_EXPERIMENTAL_CANVAS_FEATURES_NAME" desc="Name of the 'Enable experimental canvas features' lab.">
         Enable experimental canvas features
       </message>