Add space-saving overloads for DictionaryValue::SetXXX...

Add space-saving overloads for DictionaryValue::SetXXX

The problem wil passing string literals like L"harry" to const std::wstring&
parameters is that a expression temporary string object is created at the call
site.  The code to construct and destroy the implicit expression temporary
at each call site contributes to the code size.

Adding 'const wchar_t*' overloads for these commonly called functions reduces
the size of chrome.dll by 17k by moving the code to construct an destroy the
temporary into a small number of overloads.

BUG=none
TEST=none