ipc: disable AFDO on Linux

To work around a 200% (!) binary size increase caused by AFDO in
nacl_helper, locally disable it.

This may have a small performance impact, though it's likely that any
perf delta will be negligible.

Bug: 916130
Change-Id: Idf37c813215a8d5a19431499b671f7579b573277
Reviewed-on: https://chromium-review.googlesource.com/c/1396565
Commit-Queue: George Burgess <gbiv@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#620506}

ipc/BUILD.gn

@@ -103,6 +103,16 @@ component("ipc") {
     "//base",
   ]
 
+  # TODO(https://crbug.com/916130): AFDO causes a substantial size increase in
+  # nacl_helper that originates from here. This is apparently due to some
+  # mixture of inlining, CFI, and (potentially) speculative devirtualization.
+  # Work around that by locally disabling AFDO.
+  #
+  # nacl_helper is only available on Linux.
+  if (is_linux) {
+    configs -= [ "//build/config/compiler:afdo" ]
+  }
+
   if (enable_ipc_fuzzer) {
     public_configs = [ "//tools/ipc_fuzzer:ipc_fuzzer_config" ]
   }