build: Enable -fwhole-program-vtables under ThinLTO.

This feature has been supported under ThinLTO for some time now.

BUG=660216
R=thakis@chromium.org

Review-Url: https://codereview.chromium.org/2890573002
Cr-Commit-Position: refs/heads/master@{#472304}

build/config/compiler/BUILD.gn

@@ -485,16 +485,8 @@ config("compiler") {
             rebase_path("$root_out_dir/thinlto-cache", root_build_dir),
       ]
     } else {
-      # Note: ThinLTO does not currently have this feature implemented
-      # For Full LTO, it provides a measurable runtime speedup of Chrome.
-      cflags += [
-        "-flto",
-        "-fwhole-program-vtables",
-      ]
-      ldflags += [
-        "-flto",
-        "-fwhole-program-vtables",
-      ]
+      cflags += [ "-flto" ]
+      ldflags += [ "-flto" ]
 
       # Apply a lower LTO optimization level as the default is too slow.
       if (is_linux) {
@@ -504,6 +496,9 @@ config("compiler") {
       }
     }
 
+    cflags += [ "-fwhole-program-vtables" ]
+    ldflags += [ "-fwhole-program-vtables" ]
+
     # Work-around for http://openradar.appspot.com/20356002
     if (is_mac) {
       ldflags += [ "-Wl,-all_load" ]