Add move constructor and assignment operator to RefPtr class

Add move constructor and assignment operator to RefPtr class. As a result, we
can now use the implicitly defined move constructor and assignment operator for
Atomic String. Also, other classes with RefPtr data members like CString now
get a better default move constructor and assignment operator.

The release binary is ~21Kb smaller, there is no diff on the generated asm
for CString and AtomicString. However, there is a small diff for WTFString.o:
http://pastebin.com/RTzX23JV

The generated assembly for "CString WTF::String::latin1() const" and
"CString WTF::String::ascii() const" is slightly smaller, likely due to
CString's improved move copy constructor as these methods return CStrings.

Review URL: https://codereview.chromium.org/354023003

git-svn-id: svn://svn.chromium.org/blink/trunk@177066 bbb929c8-8fbe-4397-9dbb-9b2b20218538