Update Gamepad.axes / GamepadButton.value to match the latest specification

Update Gamepad.axes / GamepadButton.value to match the latest specification:
https://dvcs.w3.org/hg/gamepad/raw-file/default/gamepad.html#widl-Gamepad-axes
https://dvcs.w3.org/hg/gamepad/raw-file/default/gamepad.html#widl-GamepadButton-value

Those attributes should be of type double as per the specification but Blink
was still using float type as in older version of the specification. This can
lead to precision problem on JavaScript side.

This CL introduces temporary ENABLE_NEW_GAMEPAD_API #ifdefs to help with the
transition. Those will be removed once the Chromium side has been updated.

R=eseidel@chromium.org, bajones@chromium.org
BUG=344556
TEST=gamepad/gamepad-events-basic.html

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=174428

Review URL: https://codereview.chromium.org/280393004

git-svn-id: svn://svn.chromium.org/blink/trunk@174544 bbb929c8-8fbe-4397-9dbb-9b2b20218538