Almost finish moving core/platform/text into platform/text

This CL moves all but four files from core/platform/text into platform/text.
The remaining files do not belong in platform/text for various reasons. Future
CLs will move them somewhere appropriate.

In addition to purely moving files, I've inlined LineBreakIteratorPoolICU.h
into the one file that included it. That header declared some static storage,
which is undesirable in a header because each compilation unit that includes
the header will allocate the storage.

TBR=eseidel
BUG=297477

Review URL: https://codereview.chromium.org/26202003

git-svn-id: svn://svn.chromium.org/blink/trunk@159010 bbb929c8-8fbe-4397-9dbb-9b2b20218538