Simplify layout attribute invalidation in LayoutSVGText

Move invalidation to a new method (invalidatePositioningValues), and
also make sure to clear LayoutSVGText::m_layoutAttributes to make it
more robust.

BUG=405966, 594058

Review URL: https://codereview.chromium.org/1856393002

Cr-Commit-Position: refs/heads/master@{#385274}
4 files changed