Fix r157217: ScriptExecutionContext can be NULL, handle it gracefully.

The ASSERT was (flakily) failing in tests. This CL matches the behavior with the
"before r157217" state which didn't try to retrieve the Isolate from the
ScriptExecutionContext and thus didn't (necessarily) crash if
ScriptExecutionContext was NULL.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/23509006

git-svn-id: svn://svn.chromium.org/blink/trunk@157288 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Introduce toSVGGlyphElement(), toSVGHKernElement() and toSVGVKernElement(), and use them

As a step to clean-up static_cast<SVG*Element>, toSVGGlyph|HKern|VKernElement() are added to clean-up static_cast<SVGGlyph|HKern|VKernElement*>

BUG=

Review URL: https://chromiumcodereview.appspot.com/23705005

git-svn-id: svn://svn.chromium.org/blink/trunk@157289 bbb929c8-8fbe-4397-9dbb-9b2b20218538