Draw find-in-page text highlight markers for svg text.

This patch fixes find-in-page highlight markers in SVG, including both
<text> and <textPath>.

Previously, SVGRootInlineBox::paint and SVGInlineFlowBox::paint computed
DocumentMarker rects (this manifests as small yellow highlights on the
page's scrollbar) but didn't actually draw the highlight on the text.
This patch extracts out computeTextMatchMarkerRectForRenderer and
re-uses the logic in SVGInlineTextBox::paintTextMatchMarker which both
computes the DocumentMarker rects and draws the text highlights. A
comment has been added in InlineTextBox.h explaining how
paintTextMatchMarker works.

This patch also cleans up some messy paint() callsites which
unnecessarily created FloatPoints.

BUG=56580

Review URL: https://codereview.chromium.org/148673004

git-svn-id: svn://svn.chromium.org/blink/trunk@166368 bbb929c8-8fbe-4397-9dbb-9b2b20218538