RenderText: Access characters through m_text instead of caching data pointers separately.
<http://webkit.org/b/109357>

Reviewed by Antti Koivisto.

Go through RenderText::m_text.impl() instead of caching the character data pointer.
RenderText should never have a null String in m_text so it's safe to access impl() directly.
We have assertions for this since before.

Removing this pointer shrinks RenderText by 8 bytes, allowing it to fit into a snugger size class.
749 KB progression on Membuster3.

* rendering/RenderText.cpp:
(SameSizeAsRenderText):
(WebCore::RenderText::RenderText):
(WebCore::RenderText::setTextInternal):
* rendering/RenderText.h:
(WebCore::RenderText::is8Bit):
(WebCore::RenderText::characters8):
(WebCore::RenderText::characters16):
(WebCore::RenderText::characterAt):
(WebCore::RenderText::operator[]):
(RenderText):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142398 268f45cc-cd09-0410-ab3c-d52691b4dbfc