Don't expand <symbol> elements in <use> that are not targets

When a <use> was referencing a subtree which contained a <symbol>
element, the <symbol> would get replaced by the "replace <symbol>" part
of the <use> expansion. This would result in content being rendered that
should not (since only <symbol>s that are directly referenced by <use>
should render.)
Instead of blindly expanding <symbol> elements, replace them directly
when producing the instance clone.

Replace the old faulty test svg/custom/use-on-g-containing-symbol.svg
with a new test.

Test from: https://bugs.webkit.org/show_bug.cgi?id=154576

BUG=589682

Review URL: https://codereview.chromium.org/1736283003

Cr-Commit-Position: refs/heads/master@{#379532}

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