[SVG] Compensate for container size snapping

BoxPainter rounds its box rect coordinates to ensure optimal alignment
and minimize AA bleed.

SVGRootPainter attempts to do the same, but it only adjusts the offset.

Since int-snapping may also change the size, the SVG content transform
needs to take this residual scale into account (such that it maps onto
the full int-snapped area).

BUG=599095
[email protected],[email protected],[email protected]

Review URL: https://codereview.chromium.org/1847983002

Cr-Commit-Position: refs/heads/master@{#384580}
4 files changed