Use the line box's direction in computeGlyphOverflow

Use SVGInlineTextBox::constructTextRun in order to get the direction as
determined by the BiDi algorithm rather than the what is specified on
the element. (This should also get the right override value for similar
reasons.)

BUG=596721

Review URL: https://codereview.chromium.org/1823073002

Cr-Commit-Position: refs/heads/master@{#382655}