Fix a crash by removing an assertion in InsertionPoint::isActive().

InsertionPoint::isActive() assume the following condition always be held:
  - Given an insertion point, if insertionPoint->isInShadowTree() is true,
  then insertionPoint->containingShadowRoot() is always non-null ShadowRoot object.

But this assumption would break while processing editing command, "insert text" (The stack trace is attached at the bug), where the following conditions would be satisfied, unfortunately:
  - insertionPoint->isInShadowTree is true,
  - But the insertionPoint's treeScope()->rootNode() is a *document*, rather than a shadow root.

This patch removes the assertion and explicitly checks whether the insertionPoint->containingShadowRoot() is really non-null shadow root object or not.

This is a quick fix for the P1 crash. See the bug for further investigation.

BUG=338545
TEST=Manually checked that crash doesn't happen. The test case is really hard to minimize.

Review URL: https://codereview.chromium.org/140663008

git-svn-id: svn://svn.chromium.org/blink/trunk@166093 bbb929c8-8fbe-4397-9dbb-9b2b20218538