Remove redundancies of multiply() in FFTFrame.

There is code redundancies of multiply() in FFTFrame for every port even
if it is exactly same. Move to FFTFrame.cpp for removing redundancies.

BUG=349945

Review URL: https://codereview.chromium.org/176843021

git-svn-id: svn://svn.chromium.org/blink/trunk@168721 bbb929c8-8fbe-4397-9dbb-9b2b20218538