The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a0d5ac61931de6d57d2f7c2a619177986d4d5e0e

commit a0d5ac61931de6d57d2f7c2a619177986d4d5e0e
Author: kinuko <kinuko@chromium.org>
Date: Fri Nov 04 08:49:59 2016

Skip base::string16 if not necessary for WebString <-> ASCII conversion

One common way to get ASCII string from WebString is to do
base::UTF16ToASCII(webString)

but this always goes through string16 even when webString only
contains ASCII and is already a 8-bit string (which is expected).

This patch adds following WebString methods to help optimizing
WebString <-> ASCII conversions:

- WebString::ascii() ... returns ascii std::string, which skips string16 conversion
  for 8-bit cases. Internally DCHECKs if it contains non-ascii chars
- WebString::containsOnlyASCII() ... should be more efficient than calling
  base::IsStringASCII() on WebString (which incurs string16 conversion)
- WebString::fromASCII() ... does same as fromLatin1() but with DCHECK

Split from https://codereview.chromium.org/2444873002/

BUG= 661895 

Review-Url: https://codereview.chromium.org/2469353003
Cr-Commit-Position: refs/heads/master@{#429834}

[modify] https://crrev.com/a0d5ac61931de6d57d2f7c2a619177986d4d5e0e/content/child/simple_webmimeregistry_impl.cc
[modify] https://crrev.com/a0d5ac61931de6d57d2f7c2a619177986d4d5e0e/content/renderer/cache_storage/cache_storage_dispatcher.cc
[modify] https://crrev.com/a0d5ac61931de6d57d2f7c2a619177986d4d5e0e/content/renderer/render_thread_impl.cc
[modify] https://crrev.com/a0d5ac61931de6d57d2f7c2a619177986d4d5e0e/content/renderer/render_view_browsertest.cc
[modify] https://crrev.com/a0d5ac61931de6d57d2f7c2a619177986d4d5e0e/content/renderer/render_view_impl.cc
[modify] https://crrev.com/a0d5ac61931de6d57d2f7c2a619177986d4d5e0e/content/renderer/renderer_blink_platform_impl.cc
[modify] https://crrev.com/a0d5ac61931de6d57d2f7c2a619177986d4d5e0e/content/renderer/webclipboard_impl.cc
[modify] https://crrev.com/a0d5ac61931de6d57d2f7c2a619177986d4d5e0e/media/blink/key_system_config_selector.cc
[modify] https://crrev.com/a0d5ac61931de6d57d2f7c2a619177986d4d5e0e/media/blink/webcontentdecryptionmodulesession_impl.cc
[modify] https://crrev.com/a0d5ac61931de6d57d2f7c2a619177986d4d5e0e/media/blink/webencryptedmediaclient_impl.cc
[modify] https://crrev.com/a0d5ac61931de6d57d2f7c2a619177986d4d5e0e/third_party/WebKit/Source/platform/exported/WebString.cpp
[modify] https://crrev.com/a0d5ac61931de6d57d2f7c2a619177986d4d5e0e/third_party/WebKit/public/platform/WebString.h