Add jumbo support for Blink/core generated files

Core generated files does not use the same template as most blink core
code so it needs to be explicitly turned on (note jumbo is still by
default disabled so normal builds are not yet affected).

This saves about 8 CPU minutes for me.

Bug: 713137
Change-Id: I73ad558021b245c8742672cd3903e27dfdf48ab5
Reviewed-on: https://chromium-review.googlesource.com/563682Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: bratell at Opera <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#485232}

third_party/WebKit/Source/core/BUILD.gn

@@ -1075,7 +1075,8 @@ group("all_generators") {
 }
 
 # Compiles the code generated by the targets above.
-target(core_link_small_target_type, "core_generated") {
+jumbo_target("core_generated") {
+  target_type = core_link_small_target_type
   sources = bindings_core_v8_files
 
   # Add all sources generated by the targets above.