Add support for floating point constants in IDL files.

Additionally this doesn't generate compile time assertions to check the value of string and floating point constants, since at least until C++11 that just isn't possible.

Floating point constants will be used in the upcoming Geofencing API.

BUG=None

Review URL: https://codereview.chromium.org/334653005

git-svn-id: svn://svn.chromium.org/blink/trunk@177583 bbb929c8-8fbe-4397-9dbb-9b2b20218538