Add toWebCoreString() / toWebCoreAtomicString() overloads taking a v8::String

Add toWebCoreString() / toWebCoreAtomicString() overloads taking a v8::String
input since as lot of callers already have a v8::String or already did a
v8::Value::IsString() check. Those callers now use the faster overload instead
of using the one that takes a v8::String as input.

The new overload is faster because it does not use v8StringResource and
thus bypasses useless checks when the input is already a v8::String.

Note that the overload taking a v8::Value as input will eventually
disappear. All callers should either use V8TRYCATCH_FOR_V8STRINGRESOURCE()
macro instead of the v8::String overload depending on the input type.

R=haraken
BUG=

Review URL: https://chromiumcodereview.appspot.com/24139004

git-svn-id: svn://svn.chromium.org/blink/trunk@157828 bbb929c8-8fbe-4397-9dbb-9b2b20218538