Move StringView constructors into WTFString.h/AtomicString.h

This will allow String and AtomicString to depend on StringView for
methods and arguments, for example making operator== take a StringView
to simplify the overload set.

I also removed the:
    StringView(const void* bytes, unsigned length, bool is8Bit)
constructor since it could be called by mistake when doing:
StringView("foo", 1, 2) trying to do (offset, length) when you're
supposed to do StringView("foo" + 1, 2) instead.

This had confused me on a couple occasions where I was passing a
length and setting is8Bit instead. This constructor is only needed
by CSSParserToken so lets just inline it there.

BUG=615174

Review-Url: https://codereview.chromium.org/2140353002
Cr-Commit-Position: refs/heads/master@{#404943}
6 files changed