Convert first letter into a pseudo element.

Currently, first letter renderers are created and destroyed through the updateFirstLetters() call in
RenderBlock. This has, historically, been problematic as we may miss places where the call
was required, leading to accessing invalid memory.

This CL converts the first letter code to use a PseudoElement for the first-letter instead of
creating classes during layout.

With the PseudoElement implementation the two tests which are currently in
LayoutTests/fast/css/first-letter-removed-added.html work as expected.

BUG=391288

Review URL: https://codereview.chromium.org/571603003

git-svn-id: svn://svn.chromium.org/blink/trunk@183913 bbb929c8-8fbe-4397-9dbb-9b2b20218538