Support jumbo builds in core unit tests (except editing)

This patch makes the build system for core unit tests support jumbo
builds (unity builds) which saves roughly 60 CPU minutes (5% of the
build time) on my computer. Currently jumbo is by default disabled so
this will have no direct effect unless you have
  use_jumbo_build = true
in your gn settings.

Bug: 713137
Change-Id: I376e62fb66738cba9135d02f8079d983cfe67495
Reviewed-on: https://chromium-review.googlesource.com/575055Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#488238}

third_party/WebKit/Source/core/BUILD.gn

@@ -236,7 +236,7 @@ component("core") {
   }
 }
 
-source_set("testing") {
+jumbo_source_set("testing") {
   configs += [
     ":blink_core_pch",
     "//third_party/WebKit/Source:config",
@@ -1191,7 +1191,7 @@ fuzzer_test("text_resource_decoder_fuzzer") {
   ]
 }
 
-source_set("unit_tests") {
+jumbo_source_set("unit_tests") {
   testonly = true
   sources = [
     "animation/AnimationClockTest.cpp",