Few renaming cleanups in CSSComputedStyleDeclaration.

Replace the various patterns to name the functions inside the file (getXXX,
valueForXXX, XXXtoCSSValue) by one : valueForXXX.

Review URL: https://chromiumcodereview.appspot.com/22849006

git-svn-id: svn://svn.chromium.org/blink/trunk@156234 bbb929c8-8fbe-4397-9dbb-9b2b20218538