Add utility method for WebString to base::FilePath conversion

third_party/WebKit/Source/platform/exported/FilePathConversion.cpp

+// Copyright 2016 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.
+
+#include "public/platform/FilePathConversion.h"
+
+#include "base/files/file_path.h"
+#include "public/platform/WebString.h"
+#include "wtf/text/StringUTF8Adaptor.h"
+#include "wtf/text/WTFString.h"
+
+namespace blink {
+
+base::FilePath WebStringToFilePath(const WebString& webString)
+{
+    if (webString.isEmpty())
+        return base::FilePath();
+
+    // Depending on platforms base::FilePath can be efficiently constructed
+    // from UTF8 or UTF16.

[Nico, 2016/02/15 16:45:14]

After:

Windows:
- 8bit:
  - ascii-only: calls FromUTF8Unsafe without conversions, which then calls
UTF8ToWide()
  - full latin-1: StringUTF8Adaptor converts from latin1 to utf8, and then
UTF8ToWide() (could probably go latin1->wide directly here)

- 16bit:
  - 0 conversions

Mac, Android, CrOS:
- 8bit:
  - ascii-only: 0 conversions
  - full latin-1: 1 conversion latin1 to utf8
-16bit:
  - 0 conversions to StringPiece16 followed by a conversion utf16->utf8 (i.e. 1
total)

Linux:
- 8bit:
  - ascii-only: 0 conversions to get the string piece, then one utf8->wstring,
then elaborate SysWideToNativeMB
  - full latin-1: 1 more conversion than ascii-only
-16bit:
  - 0 to StringPiece16, then 1 to wstring, then elaborate SysWideToNativeMB

+    String str = webString;
+    if (str.is8Bit()) {
+        StringUTF8Adaptor utf8(str);
+        return base::FilePath::FromUTF8Unsafe(utf8.asStringPiece());
+    }
+    return base::FilePath::FromUTF16Unsafe(

[Nico, 2016/02/15 16:45:14]

does this branch have test coverage?

+        base::StringPiece16(str.characters16(), str.length()));
+}
+
+} // namespace blink