Add an `auto_profile_path` flag to GN to control AFDO.

build/config/compiler/compiler.gni

Index: build/config/compiler/compiler.gni
diff --git a/build/config/compiler/compiler.gni b/build/config/compiler/compiler.gni
index 5b51fafa974721771f2863984672798cb41a3646..53f27435575736ea14ed1e949f6c3e0c41f42451 100644
--- a/build/config/compiler/compiler.gni
+++ b/build/config/compiler/compiler.gni
@@ -34,6 +34,12 @@ declare_args() {
   # been set, e.g. by a toolchain_args() block.
   use_debug_fission = "default"
 
+  # AFDO (Automatic Feedback Directed Optimizer) is a form of profile-guided
+  # optimization that GCC supports. It used by ChromeOS in their official
+  # builds. To use it, set auto_profile_path to the path to a file containing
+  # the needed gcov profiling data.
+  auto_profile_path = ""

[brettw, 2016/09/27 23:52:41]

This appears in a different declare_args block in the previous file. That seems
wrong and I would have expected GN to give an error. Maybe we just want this
one?

[Dirk Pranke, 2016/09/28 00:08:34]

Oh, whoops, I forgot to back this change out. I did this change and then the
BUILD.gn one, at which point I realized we didn't need to expose the arg in the
.gni.

+
   # Tell VS to create a PDB that references information in .obj files rather
   # than copying it all. This should improve linker performance. mspdbcmf.exe
   # can be used to convert a fastlink pdb to a normal one.