Disable blink pch for jumbo builds

pch makes jumbo builds slower instead of faster since each
pch file will only be used 1-3 times instead of many times, and
in Blink the pch files contain a lot of code so creating them
takes a long time.

This saves about 20% of the Blink build time on Windows.

Bug: 809010
Change-Id: Ia512646110572e4939e8e3cc1a9cd946b780cb97
Reviewed-on: https://chromium-review.googlesource.com/962785Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#543335}

third_party/WebKit/Source/core/BUILD.gn

@@ -53,7 +53,14 @@ config("core_include_dirs") {
 import("//build/config/pch.gni")
 
 config("blink_core_pch") {
-  if (enable_precompiled_headers) {
+  # Add precompiled header (pch) support when enabled, possible and
+  # making sense.
+
+  # Precompiled headers are disabled for jumbo builds because they
+  # slow down the build since each pch file is only used 1-3
+  # times. Not nearly enough to make up for the long time spent
+  # creating them.
+  if (enable_precompiled_headers && !use_jumbo_build) {
     if (is_win) {
       # This is a string rather than a file GN knows about. It has to match
       # exactly what's in the /FI flag below, and what might appear in the