Add support for type checking of floating point attributes

Just do the same thing for value passed to setter(if (!std::isfinite) ...)
as for method arguments.

Add examples in SourceBuffer for checking.

Followup to:
Add support for type checking of floating point arguments as [TypeChecking=Unrestricted]
https://codereview.chromium.org/259773008/

This does *not* add TypeChecking throughout SourceBuffer, only enough for testing; Fredrik is doing full type checking in:
Apply TypeChecking to MediaSource IDLs
https://codereview.chromium.org/266453005/

R=haraken
BUG=354298
TEST=http/tests/media/media-source/mediasource-timestamp-offset.html
TEST=http/tests/media/media-source/mediasource-appendwindow.html

Review URL: https://codereview.chromium.org/265753003

git-svn-id: svn://svn.chromium.org/blink/trunk@173064 bbb929c8-8fbe-4397-9dbb-9b2b20218538