Remove rounding in RenderGeometryMap.

When working with FloatPoints in mapToContainer we currently round the
accumulated offset before adding to the current point. This can lead to
incorrect results when we add the two values together.

This CL removes the rounding call on accumulatedOffset.

BUG=371640

Review URL: https://codereview.chromium.org/302023003

git-svn-id: svn://svn.chromium.org/blink/trunk@175105 bbb929c8-8fbe-4397-9dbb-9b2b20218538