Rename HACK_FORCE_TEXT_AUTOSIZING_ON_DESKTOP

Because this build time flag is only used for debugging purpose,
rename it to DEBUG_TEXT_AUTOSIZING_ON_DESKTOP.

Review URL: https://codereview.chromium.org/720203002

git-svn-id: svn://svn.chromium.org/blink/trunk@185335 bbb929c8-8fbe-4397-9dbb-9b2b20218538