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

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 7f57d912d6f1ec60650af9ce04dc3bf61fc37683..e28e48d4aec90171032798060c398b80f8eab23f 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -487,16 +487,8 @@ config("compiler") {
         ldflags += [ "-Wl,-plugin-opt,jobs=8" ]
       }
     } 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) {
@@ -510,6 +502,9 @@ config("compiler") {
       }
     }
 
+    cflags += [ "-fwhole-program-vtables" ]
+    ldflags += [ "-fwhole-program-vtables" ]
+
     # Work-around for http://openradar.appspot.com/20356002
     if (is_mac) {

[Nico, 2017/05/16 19:51:19]

Huh, why do we have mac stuff in here? We don't use LTO on mac, do we?

[pcc1, 2017/05/16 19:57:33]

I think this is from some early work I did to try to support LTO on Mac
(crbug.com/471146). I'd be fine with removing it, we can always bring it back
later.

       ldflags += [ "-Wl,-all_load" ]