ThinLTO: limit link / codegen parallelism to 16.

Based on the experiments, ThinLTO does not scale beyond that,
and the default setting is the number of cores on a machine, which
is often 32 or more for Chrome devs, where ThinLTO is slower than at 16:
https://docs.google.com/spreadsheets/d/18vi9p8ffIYNVPTyxtJwr-YrP4WJRbaQr_2nZ1AKKBs4/edit?usp=sharing

BUG=645295

Review-Url: https://codereview.chromium.org/2341983002
Cr-Commit-Position: refs/heads/master@{#418789}

build/config/compiler/BUILD.gn

@@ -434,7 +434,13 @@ config("compiler") {
   if (!is_debug && (allow_posix_link_time_opt || is_cfi) && !is_nacl) {
     if (use_thin_lto) {
       cflags += [ "-flto=thin" ]
-      ldflags += [ "-flto=thin" ]
+      ldflags += [
+        "-flto=thin",
+
+        # Limit the parallelism to the sweet spot on most of the machines.
+        # As of now, ThinLTO does not scale beyond 16 cores anyway.
+        "-Wl,-plugin-opt,jobs=16",
+      ]
     } else {
       # Note: ThinLTO does not currently have this feature implemented
       # For Full LTO, it provides a measurable runtime speedup of Chrome.