Use _self to represnet self-navigation, like in the HTML5 spec

Use _self to represnet self-navigation, like in the HTML5 spec
https://bugs.webkit.org/show_bug.cgi?id=83834

Reviewed by Eric Seidel.

This patch aligns the internal state representation of the FrameLoader
more closely with the HTML5 spec.  Both the null string and "_self"
represent self-navigation, but the HTML5 spec distinguishes them when
navigating seamless iframes.  In that case, "_self" is explicit
self-navigation, whereas a null string will get redirected to the
"master" frame (the closest enclosing non-seamless frame).

This patch shouldn't have any observable behavior changes, but it will
be useful when we merge in the seamless branch from GitHub.
(See https://github.com/eseidel/webkit/commit/df0e71fcacdb0e7e45fe669afe1314cec5b76523
which contains this change.)

Once the seamless branch lands, this change will be tested by a number
of tests in
https://github.com/eseidel/webkit/tree/seamless/LayoutTests/fast/frames/seamless
including seamless-hyperlink.html.

* loader/FrameLoader.cpp:
(WebCore::FrameLoader::loadURLIntoChildFrame):
(WebCore::FrameLoader::loadURL):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@114077 268f45cc-cd09-0410-ab3c-d52691b4dbfc