kThemePackVersion comment should mention theme cache flushing.

chrome/browser/themes/browser_theme_pack.cc

Index: chrome/browser/themes/browser_theme_pack.cc
diff --git a/chrome/browser/themes/browser_theme_pack.cc b/chrome/browser/themes/browser_theme_pack.cc
index 3f2837e8988911c7efee1b58bf0d6f8d27a56a43..8cc62dd278d900e58d2a0c911ecf4a9d3fd47224 100644
--- a/chrome/browser/themes/browser_theme_pack.cc
+++ b/chrome/browser/themes/browser_theme_pack.cc
@@ -48,7 +48,8 @@ namespace {
 
 // Version number of the current theme pack. We just throw out and rebuild
 // theme packs that aren't int-equal to this. Increment this number if you
-// change default theme assets.
+// change default theme assets or if you need themes to recreate their generated
+// images (which are cached).
 const int kThemePackVersion = 41;
 
 // IDs that are in the DataPack won't clash with the positive integer