[DevTools] Extract UTF8<->UTF16 routines from string-16{.h,.cc}.

This will allow me to reuse the routines for the
inspector_protocol/encoding/encoding_test, rather than copy them.
I've already verified that this works.

Change-Id: I5b61e4f48c2d00d9d0a5305c9790093c66b3c765
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1600360
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61355}

src/inspector/BUILD.gn

@@ -80,8 +80,20 @@ v8_header_set("inspector_test_headers") {
   ]
 }
 
+v8_source_set("inspector_string_conversions") {
+  sources = [
+    "v8-string-conversions.cc",
+    "v8-string-conversions.h",
+  ]
+  configs = [ "../..:internal_config_base" ]
+  deps = [
+    "../..:v8_libbase",
+  ]
+}
+
 v8_source_set("inspector") {
   deps = [
+    ":inspector_string_conversions",
     "../..:v8_version",
     "../../third_party/inspector_protocol:encoding",
   ]

src/inspector/string-16.h

@@ -30,6 +30,7 @@ class String16 {
   String16(const char* characters);   // NOLINT(runtime/explicit)
   String16(const char* characters, size_t size);
   explicit String16(const std::basic_string<UChar>& impl);
+  explicit String16(std::basic_string<UChar>&& impl);
 
   String16& operator=(const String16&) V8_NOEXCEPT = default;
   String16& operator=(String16&&) V8_NOEXCEPT = default;

src/inspector/v8-string-conversions.h

+// Copyright 2019 the V8 project 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 V8_INSPECTOR_V8_STRING_CONVERSIONS_H_
+#define V8_INSPECTOR_V8_STRING_CONVERSIONS_H_
+
+#include <string>
+
+// Conversion routines between UT8 and UTF16, used by string-16.{h,cc}. You may
+// want to use string-16.h directly rather than these.
+namespace v8_inspector {
+std::basic_string<uint16_t> UTF8ToUTF16(const char* stringStart, size_t length);
+std::string UTF16ToUTF8(const uint16_t* stringStart, size_t length);
+}  // namespace v8_inspector
+
+#endif  // V8_INSPECTOR_V8_STRING_CONVERSIONS_H_