[mojo] use forward declaration for WTF::DefaultHash

This CL reduced total include for ip_address_space.mojom-blink-forward.h
from 0.72MB to 0.22MB.

Bug: 862064
Change-Id: I2371e1513a194ecd0275590522522f30983c0527
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609737
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#659076}

mojo/public/tools/bindings/generators/cpp_templates/module-forward.h.tmpl

@@ -100,7 +100,12 @@ using {{enum|get_name_for_kind(flatten_nested_kind=True)}} = mojo::NativeEnum;
 {%- from "enum_macros.tmpl" import enum_hash_blink_forward%}
 {%- if for_blink %}
 {%-   if all_enums %}
-#include "third_party/blink/renderer/platform/wtf/hash_functions.h"
+
+namespace WTF {
+template <typename T>
+struct DefaultHash;
+}
+
 {%-   endif %}
 
 {%-   for enum in all_enums %}