Minor refactoring to avoid redundant calls to VisibleSelection::visibleStart()

We can avoid redundant calls to visibleStart() which indirectly calls
several functions like canonicalPosition, isCandidate, upstream,
downstream, etc. by using a local variable.

Current code already makes use of a local variable to store the VisiblePosition
returned by visibleStart(). Just preponing the creation of this
local variable would avoid few more redundant calls.

Review URL: https://codereview.chromium.org/132903007

git-svn-id: svn://svn.chromium.org/blink/trunk@165561 bbb929c8-8fbe-4397-9dbb-9b2b20218538