Fixed the insertion handle showing issue on readonly element.

Insertion handle is showing when we long press on readonly input
elements, which is not correct. This change takes care of correcting
it by counter measuring inside isEditableNode() API.

BUG=

Review URL: https://codereview.chromium.org/660343003

Cr-Commit-Position: refs/heads/master@{#300636}
3 files changed