Prepare for building with enable_app_list=0 on Desktop

chrome/browser/ui/BUILD.gn

Index: chrome/browser/ui/BUILD.gn
diff --git a/chrome/browser/ui/BUILD.gn b/chrome/browser/ui/BUILD.gn
index 32cb2a3610fc0a4ad18471df3efef45e27cc9fc4..989723484f2abd3467d020aaa683bc62246981e2 100644
--- a/chrome/browser/ui/BUILD.gn
+++ b/chrome/browser/ui/BUILD.gn
@@ -450,7 +450,6 @@ source_set("ui") {
       "//third_party/iaccessible2",
       "//third_party/isimpledom",
       "//third_party/wtl",
-      "//ui/app_list",
       "//ui/base/ime",
       "//ui/events:dom_keycode_converter",
     ]
@@ -462,6 +461,9 @@ source_set("ui") {
         "webui/help/version_updater_basic.h",
       ]
     }
+    if (enable_app_list) {
+      deps += [ "//ui/app_list" ]
+    }
   } else {  # 'OS!="win"
     if (toolkit_views) {
       public_deps += [
@@ -514,6 +516,18 @@ source_set("ui") {
                              ".",
                              "//chrome")
     }
+    if (is_desktop_linux) {

[Matt Giuca, 2016/05/03 07:31:16]

Can you explain what this is doing?

[tapted, 2016/05/04 09:55:43]

This is adding the app_list_linux_sources - i.e. those 4 files from
chrome_browser_ui.gypi for desktop linux but not ChromeOS --
app_list_{service_,}linux.{cc,h} -- which were previously in 

# Desktop Linux. Assume aura/ash/views/x11.
'chrome_browser_ui_desktop_linux_sources': [
  /* here */


I think this fits the common pattern, but holler if you think it needs a comment
in the .gn file

[Matt Giuca, 2016/05/05 00:26:30]

Acknowledged.

+      sources +=
+          rebase_path(gypi_values.chrome_browser_ui_app_list_linux_sources,
+                      ".",
+                      "//chrome")
+    }
+    if (!is_mac) {
+      sources +=
+          rebase_path(gypi_values.chrome_browser_ui_app_list_views_sources,
+                      ".",
+                      "//chrome")
+    }
     deps += [ "//ui/app_list" ]
   } else {
     sources += rebase_path(gypi_values.chrome_browser_ui_non_app_list_sources,