https://bugs.webkit.org/show_bug.cgi?id=127161

Reviewed by Sam Weinig.

Handle OpaqueJSString::m_string being either 8-bit or 16-bit and add extra
code paths for the 8-bit cases.

Unfortunately, JSStringGetCharactersPtr is still expected to return a 16-bit character pointer.
Handle this by storing a separate 16-bit string and initializing it on demand when JSStringGetCharactersPtr
is called. This has the nice side effect of making JSStringGetCharactersPtr thread-safe when it wasn't before.
(In theory, someone could have a JSStringRef backed by an 8-bit string and call JSStringGetCharactersPtr on it
causing an unsafe upconversion to a 16-bit string).

* API/JSStringRef.cpp:
(JSStringGetCharactersPtr):
Call OpaqueJSString::characters.

(JSStringGetUTF8CString):
Add a code path that handles 8-bit strings.

(JSStringIsEqual):
Call OpaqueJSString::equal.

* API/JSStringRefCF.cpp:
(JSStringCreateWithCFString):
Reformat the code to use an early return instead of putting most of the code inside the body of an if statement.

(JSStringCopyCFString):
Create an 8-bit CFStringRef if possible.

* API/OpaqueJSString.cpp:
(OpaqueJSString::create):
Use nullptr.

(OpaqueJSString::~OpaqueJSString):
Free m_characters.

(OpaqueJSString::characters):
Do the up-conversion and store the result in m_characters.

(OpaqueJSString::equal):
New helper function.

* API/OpaqueJSString.h:
(OpaqueJSString::is8Bit):
New function that returns whether a string is 8-bit or not.

(OpaqueJSString::characters8):
(OpaqueJSString::characters16):
Add getters.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@162185 268f45cc-cd09-0410-ab3c-d52691b4dbfc