Add 'printing' flag to PaintInfo

This adds a new paint flag - PaintInfo::printing() - to be preferred
before Document::printing() (and GC::printing()) during paint.
These flags should always have the same value.
The users of GC::printing() were the "PDF URL rect" code, SVG text
painting (to disable shadows) and TextPainter.
In the last case, the check can be dropped entirely because
TextPainter::*PaintingStyle explicitly clears |Style::shadow| if
|isPrinting| is true.
Rearrange code in PrintContextTest::printSinglePage to better reflect
the flow of actual printing.

BUG=424655

Review URL: https://codereview.chromium.org/1236183003

git-svn-id: svn://svn.chromium.org/blink/trunk@199034 bbb929c8-8fbe-4397-9dbb-9b2b20218538
20 files changed