Rename variables in ReplaceSelectionCommand::doApply to improve readability

This patch renames variables |insertionBlock| and |startBlock| in
ReplaceSelectionCommand::doApply to |enclosingBlockOfInsertionPos| and
|enclosingBlockOfVisibleStart| to improve readability.

This patch is preparation of http://crrev.com/352943002

BUG=n/a
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/370573004

git-svn-id: svn://svn.chromium.org/blink/trunk@177507 bbb929c8-8fbe-4397-9dbb-9b2b20218538