Consolidate input conversion math

These conversions were done in multiple places. Put them all in helper
functions.

BUG=133097

Review URL: https://codereview.chromium.org/669303004

git-svn-id: svn://svn.chromium.org/blink/trunk@184385 bbb929c8-8fbe-4397-9dbb-9b2b20218538