Web Animations CSS: Fix handling of background-color: initial

This patch fixes our handling of background-color: initial. We currently
treat all invalid colors as currentColor, but this means transparent for
background-color instead. This would lead to strange transitions when we
changed background-color from e.g. initial to transparent, which doesn't
actually change the computed style.

BUG=328305

Review URL: https://codereview.chromium.org/101833005

git-svn-id: svn://svn.chromium.org/blink/trunk@163938 bbb929c8-8fbe-4397-9dbb-9b2b20218538
IDL compiler: [Constructor] overloading

This completes [Constructor].

Relatively long, b/c need to support list of constructors, but most of
logic already handled in method overloading.

A few notes:
* Also simplify Perl logic
* Some stub values (e.g., argument['is_nullable'] = False)
  b/c needed by overload resolution algorithm.
* moved constructor macro into methods.cpp b/c otherwise cyclic import
  between interface_base.cpp and interface.cpp.
* interface.cpp diff looks weird b/c it replaces the actual constructor
  method with the overload callback.

BUG=239771

Review URL: https://codereview.chromium.org/112303003

git-svn-id: svn://svn.chromium.org/blink/trunk@163939 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove the ENCRYPTED_MEDIA_V2 guard as it is always set.

Review URL: https://codereview.chromium.org/100103008

git-svn-id: svn://svn.chromium.org/blink/trunk@163940 bbb929c8-8fbe-4397-9dbb-9b2b20218538