Remove bad assertion in SVGSMILElement's constructQualifiedName()

Remove bad assertion in SVGSMILElement's constructQualifiedName() by passing
IGNORE_EXCEPTION instead ASSERT_NO_EXCEPTION to Document::parseQualifiedName().
The constructQualifiedName() function handles parseQualifiedName() failures
propertly by returning early so there is no reason to assert on parsing
failures.

R=pdr@chromium.org, eseidel@chromium.org
BUG=368325
TEST=svg/custom/bad-attributeName-crash.html

Review URL: https://codereview.chromium.org/271693003

git-svn-id: svn://svn.chromium.org/blink/trunk@173564 bbb929c8-8fbe-4397-9dbb-9b2b20218538