[Extensions] Small cleanup of extension_install_prompt.*

BUG=567839

Review URL: https://codereview.chromium.org/1575653002

Cr-Commit-Position: refs/heads/master@{#368925}
diff --git a/chrome/browser/extensions/external_install_error.cc b/chrome/browser/extensions/external_install_error.cc
index fc3c50e..02b6e5b 100644
--- a/chrome/browser/extensions/external_install_error.cc
+++ b/chrome/browser/extensions/external_install_error.cc
@@ -31,6 +31,7 @@
 #include "extensions/common/extension.h"
 #include "ui/base/l10n/l10n_util.h"
 #include "ui/gfx/image/image.h"
+#include "ui/gfx/image/image_skia.h"
 #include "ui/gfx/image/image_skia_operations.h"
 
 namespace extensions {