Make gtk2/gtk3 compile time switchable.

chrome/browser/ui/libgtk2ui/BUILD.gn

Index: chrome/browser/ui/libgtk2ui/BUILD.gn
diff --git a/chrome/browser/ui/libgtk2ui/BUILD.gn b/chrome/browser/ui/libgtk2ui/BUILD.gn
index 595d381887ca2f77ead3fc8e6b84e1bd22e27198..2f65b659c2c806be523bc48d318e66d76dbe7f55 100644
--- a/chrome/browser/ui/libgtk2ui/BUILD.gn
+++ b/chrome/browser/ui/libgtk2ui/BUILD.gn
@@ -5,6 +5,7 @@
 assert(is_linux, "This file should only be referenced on Linux")
 
 import("//build/config/features.gni")
+import("//build/config/ui.gni")
 
 component("libgtk2ui") {
   sources = [
@@ -63,14 +64,10 @@ component("libgtk2ui") {
   }
   defines = [ "LIBGTK2UI_IMPLEMENTATION" ]
 
-  configs += [
-    "//build/config/linux:gconf",
-  ]
+  configs += [ "//build/config/linux:gconf" ]
 
   if (use_cups) {
-    configs += [
-      "//printing:cups",
-    ]
+    configs += [ "//printing:cups" ]
   }
 
   # GTK2 pulls pangoft2 as dependency, and pangoft2 depends on harfbuzz.
@@ -92,6 +89,10 @@ component("libgtk2ui") {
 
         # G_STATIC_ASSERT uses a typedef as a static_assert.
         "-Wno-unused-local-typedef",
+
+        # When compiling with GTK3, some of the functions have deprecation
+        # warnings.
+        "-Wno-deprecated-declarations",

[knthzh, 2015/08/24 20:52:01]

This isn't necessary anymore. I found out that gtk lets me selectively silence
warnings for when use of deprecated functions is unavoidable:
https://developer.gnome.org/Deprecations/
Sorry for the trouble.

[Nico, 2015/08/25 00:23:52]

Cool, remove this then I guess.

       ]
     }
   }
@@ -104,8 +105,6 @@ component("libgtk2ui") {
     "//base",
     "//base/third_party/dynamic_annotations",
     "//base:i18n",
-    "//build/config/linux/gtk",
-    "//build/config/linux/gtk:gtkprint",
     "//chrome/app/theme:theme_resources",
     "//chrome:extra_resources",
     "//chrome:resources",
@@ -129,4 +128,16 @@ component("libgtk2ui") {
     "//ui/strings",
     "//ui/views",
   ]
+
+  if (use_gtk3) {
+    deps += [
+      "//build/config/linux/gtk3",
+      "//build/config/linux/gtk3:gtkprint3",
+    ]
+  } else {
+    deps += [
+      "//build/config/linux/gtk2",
+      "//build/config/linux/gtk2:gtkprint2",
+    ]
+  }
 }