Refactor SVGTextLayoutAttributesBuilder::collectTextPositioningElements

This moves the creation of a TextPosition element for the <text> into
collectTextPositioningElements too, getting rid of the special case.

BUG=594058

Review URL: https://codereview.chromium.org/1878583002

Cr-Commit-Position: refs/heads/master@{#386622}
1 file changed