Cleanup ASSERTS that trigger layout and add FIXME for isEditablePosition

isEditablePosition by default triggers layout. Ideally, the default is
no layout occurs, and the caller requests that behavior only if they
need it. In the meantime, there were 3 asserts that triggered layout
that way, which makes Debug behave differently than release. This
patch fixes that and adds some FIXMEs.

Review URL: https://codereview.chromium.org/162963002

git-svn-id: svn://svn.chromium.org/blink/trunk@167122 bbb929c8-8fbe-4397-9dbb-9b2b20218538