Extensions Web UI: Only build optimized page when optimize_webui=true

There is no reason to build the :build, :flattened_resources, or :unpak
targets when optimize_webui=false.

Bug: 1004967
Change-Id: Ib7e2cf5896279ccb984701987519d9e311944d40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829753
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701233}

chrome/browser/resources/extensions/BUILD.gn

@@ -7,10 +7,11 @@ import("//third_party/closure_compiler/compile_js.gni")
 import("//tools/grit/grit_rule.gni")
 import("../optimize_webui.gni")
 
-extensions_pak_file = "extensions_resources.pak"
-unpak_folder = "extensions_resources.unpak"
+if (optimize_webui) {
+  extensions_pak_file = "extensions_resources.pak"
+  unpak_folder = "extensions_resources.unpak"
 
-optimize_webui("build") {
+  optimize_webui("build") {
     host = "extensions"
     html_in_files = [ "extensions.html" ]
     html_out_files = [ "vulcanized.html" ]
@@ -22,18 +23,18 @@ optimize_webui("build") {
     deps = [
       ":unpak",
     ]
-}
+  }
 
-unpak("unpak") {
+  unpak("unpak") {
     pak_file = extensions_pak_file
     out_folder = unpak_folder
 
     deps = [
       ":flattened_resources",
     ]
-}
+  }
 
-grit("flattened_resources") {
+  grit("flattened_resources") {
     source = "extensions_resources.grd"
 
     # The .grd contains references to generated files.
@@ -47,6 +48,7 @@ grit("flattened_resources") {
       extensions_pak_file,
     ]
     output_dir = "$root_gen_dir/chrome/browser/resources/extensions"
+  }
 }
 
 group("closure_compile") {