Editing - Code refactoring to avoid several calls to rendererIsEditable() in loop

In startOfParagraph() and endOfParagraph(), rendererIsEditable() is called
repeatedly in loop for a node that remains unchanged throughout the function.
This is avoided by using a local variable.

TEST=No test added, since it is a refactoring. All the layout tests have been successfully executed.

Review URL: https://codereview.chromium.org/95493004

git-svn-id: svn://svn.chromium.org/blink/trunk@163268 bbb929c8-8fbe-4397-9dbb-9b2b20218538