Mojo: Add ArrayTraitsWebVector helpers

A planned change will use this from a Blink variant's mojom traits
to natively interact with variables of type WebVector.

Bug: 717812
Change-Id: I844d189c601d3a6310034abeebe51b66a36b3bc5
Reviewed-on: https://chromium-review.googlesource.com/c/1282211Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Chase Phillips <cmp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600068}

mojo/public/cpp/bindings/BUILD.gn

@@ -208,6 +208,7 @@ if (!is_ios) {
   # TODO(yzshen): crbug.com/617718 Consider moving this into blink.
   source_set("wtf_support") {
     sources = [
+      "array_traits_web_vector.h",
       "array_traits_wtf_vector.h",
       "lib/string_traits_wtf.cc",
       "lib/wtf_clone_equals_util.h",
@@ -219,6 +220,7 @@ if (!is_ios) {
 
     public_deps = [
       ":bindings",
+      "//third_party/blink/public:blink_headers",
       "//third_party/blink/renderer/platform:platform_export",
       "//third_party/blink/renderer/platform/wtf",
     ]

mojo/public/cpp/bindings/DEPS

 include_rules = [
+  "+third_party/blink/public/platform/web_vector.h",
   "+third_party/blink/renderer/platform/wtf",
 ]

mojo/public/cpp/bindings/array_traits_web_vector.h

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MOJO_PUBLIC_CPP_BINDINGS_ARRAY_TRAITS_WEB_VECTOR_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_ARRAY_TRAITS_WEB_VECTOR_H_
+
+#include "mojo/public/cpp/bindings/array_traits.h"
+#include "third_party/blink/public/platform/web_vector.h"
+
+namespace mojo {
+
+template <typename U>
+struct ArrayTraits<blink::WebVector<U>> {
+  using Element = U;
+
+  static bool IsNull(const blink::WebVector<U>& input) {
+    // blink::WebVector<> is always converted to non-null mojom array.
+    return false;
+  }
+
+  static void SetToNull(blink::WebVector<U>* output) {
+    // blink::WebVector<> doesn't support null state. Set it to empty instead.
+    output->Clear();
+  }
+
+  static size_t GetSize(const blink::WebVector<U>& input) {
+    return input.size();
+  }
+
+  static U* GetData(blink::WebVector<U>& input) { return input.Data(); }
+
+  static const U* GetData(const blink::WebVector<U>& input) {
+    return input.Data();
+  }
+
+  static U& GetAt(blink::WebVector<U>& input, size_t index) {
+    return input[index];
+  }
+
+  static const U& GetAt(const blink::WebVector<U>& input, size_t index) {
+    return input[index];
+  }
+
+  static bool Resize(blink::WebVector<U>& input, size_t size) {
+    // WebVector DCHECKs if the new size is larger than capacity().  Call
+    // reserve() first to be safe.
+    input.reserve(size);
+    input.resize(size);
+    return true;
+  }
+};
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_ARRAY_TRAITS_WEB_VECTOR_H_