[MIPS] Correct double conversion define for MIPSEL

Small change as a follow up to r157035 that broke Chromium build for MIPS.

BUG= r157035 broke Chromium build for MIPS arch

Review URL: https://chromiumcodereview.appspot.com/23658018

git-svn-id: svn://svn.chromium.org/blink/trunk@157235 bbb929c8-8fbe-4397-9dbb-9b2b20218538