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

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 4bbc34f4e7d0cc77a69d13856bbd843c116a0dab..e54ff69182706c4f940be7f78c8c540af43a8716 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -80,6 +80,12 @@ declare_args() {
   # of binutils can cause linker warning.
   # See: https://bugs.chromium.org/p/chromium/issues/detail?id=457359
   fatal_linker_warnings = true
+
+  # 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 = ""
 }
 
 if (is_clang && !is_nacl) {
@@ -1479,6 +1485,18 @@ config("default_optimization") {
   }
 }
 
+# GCC supports a form of profile-guided optimization called AFDO, which
+# is used by ChromeOS in their official builds. However,
+# //base/allocator:tcmalloc currently doesn't work correctly with AFDO
+# so we provide separate config so that the flag can be disabled per-target.
+# TODO(crbug.com/633719): Remove this config once tcmalloc works with AFDO
+# or we remove tcmalloc or we stop using AFDO.
+config("afdo") {
+  if (auto_profile_path) {
+    cflags = [ "-fauto-profile=${auto_profile_path}" ]
+  }
+}
+
 # Symbols ----------------------------------------------------------------------
 
 # The BUILDCONFIG file sets the "default_symbols" config on targets by