Extract the glyph picking and placing code.

Extract the glyph picking and placing code.

There is a common piece of code which finds and positions glyphs and is used in four places. Some places copied the code, some places added callbacks. Here is a list of code:

SkDraw::drawPosText
GrAtlasTextContext::internalDrawBMPPosText
GrAtlasTextContext::internalDrawDFPosText
SkXPSDevice::drawPosText

This only extracts the code from SkDraw::drawPosText. I would like to use it in the other three places. I think this code is performance neutral.

BUG=skia:

Review URL: https://codereview.chromium.org/1420973005