Update build comment with results of analysis.

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 2036b3451904718854386ad7b6c6013ebf768033..be1cb35979a498e132a7bacda87fa165f148289d 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -643,10 +643,20 @@ template("component") {
     # When there are no sources defined, use a source set to avoid creating
     # an empty static library (which generally don't work).
     #
-    # TODO(brettw) remove the Android condition or comment why it needs to be
-    # kept after some analysis. Source sets vs. static libraries seem to be
-    # causing some performance differences. As part of the analysis for
-    # http://crbug.com/619593 we're testing source sets for components again.
+    # When we changed components to default from source sets to static
+    # libraries, an Android benchmark regressed slightly
+    # (https://crbug.com/619593). We don't have a good theory on why this might
+    # be since theoretically it should be the same. It could be something as
+    # silly as random code locality luck.
+    #
+    # There seems to be no build-time performance hit to using source sets on
+    # Android (the normal reason for defaulting to static libraries), so we
+    # make the default on Android to be source set.
+    #
+    # If it's been a long time since this was added and you're skeptical,
+    # please feel free to remove the Android exception and see if any
+    # benchmarks obviously regress. If not, it would be great to standardize
+    # with the rest of the platforms.
     _component_mode = "source_set"
   } else {
     _component_mode = "static_library"