Do not crash when calling setRangeText() for an INPUT with author ShadowRoots.

We need to skip any code for innerTextValue when an element has author
ShadowRoots.
Also, this CL adds assertions to setInnerTextValue and innerTextValue.

BUG=345456
TEST=automated

Review URL: https://codereview.chromium.org/177093023

git-svn-id: svn://svn.chromium.org/blink/trunk@168098 bbb929c8-8fbe-4397-9dbb-9b2b20218538