Separate repaint and layout requirements of StyleDifference (Step 1)

Previously StyleDifference was an enum that proximately bigger values
imply smaller values (e.g. StyleDifferenceLayout implies
StyleDifferenceRepaint). This causes unnecessary repaints in some cases
on layout change.

Convert StyleDifference to a structure containing relatively independent
flags.

This change doesn't directly improve the result, but can make further
repaint optimizations possible.

Step 1 doesn't change any functionality. RenderStyle still generate the
legacy StyleDifference enum when comparing styles and convert the result
to the new StyleDifference. Implicit requirements are not handled during
the conversion.

Converted call sites to use the new StyleDifference according to the
following conversion rules:
- diff == StyleDifferenceEqual (&& !context) => diff.hasNoChange()
- diff == StyleDifferenceRepaint => diff.needsRepaintObjectOnly()
- diff == StyleDifferenceRepaintLayer => diff.needsRepaintLayer()
- diff == StyleDifferenceRepaint || diff == StyleDifferenceRepaintLayer => diff.needsRepaintLayer()
- diff >= StyleDifferenceRepaint => diff.needsRepaint() || diff.needsLayout()
- diff >= StyleDifferenceRepaintLayer => diff.needsRepaintLayer() || diff.needsLayout()
- diff > StyleDifferenceRepaintLayer => diff.needsLayout()
- diff == StyleDifferencePositionedMovementLayoutOnly => diff.needsPositionedMovementLayoutOnly()
- diff == StyleDifferenceLayout => diff.needsFullLayout()

BUG=358460
TEST=All existing layout tests.
R=eseidel@chromium.org, esprehn@chromium.org, jchaffraix@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=171983

Review URL: https://codereview.chromium.org/236203020

git-svn-id: svn://svn.chromium.org/blink/trunk@172331 bbb929c8-8fbe-4397-9dbb-9b2b20218538