[Closure Externs] Generate closure compiler externs as part of compile

chrome/browser/extensions/BUILD.gn

Index: chrome/browser/extensions/BUILD.gn
diff --git a/chrome/browser/extensions/BUILD.gn b/chrome/browser/extensions/BUILD.gn
index c647868f4bc8ce46062c624af055646afce4c9de..1480cf722122858cdffac8fed4d53d95a3149cea 100644
--- a/chrome/browser/extensions/BUILD.gn
+++ b/chrome/browser/extensions/BUILD.gn
@@ -826,6 +826,7 @@ static_library("extensions") {
     "//chrome/common",
     "//chrome/common/extensions/api:api_registration",
     "//chrome/common/extensions/api:extensions_features",
+    "//chrome/common/extensions/api:api_externs",

[Devlin, 2016/12/15 18:56:40]

This causes the externs to be built as part of chrome.  Really, we just need to
do it as part of running closure compiler.  Is there a good place to put them
there?

     "//chrome/common/safe_browsing:proto",
     "//chrome/installer/util:with_no_strings",
     "//components/app_modal",