Make AMP Layouts first class in the validator.

[powdercloud]

This implements the logic in src/custom-element.js, applyLayout_.

validator.proto:

• Introduce AmpLayout, a specification per TagSpec.
  When added, AmpLayout takes over responsibility for
  validating the four style attributes width,height,layout,sizes.
• AmpLayout::Layout is an enum with the layouts.
• I've modeled the natural dimensions with two booleans:
  defines_default_width and defines_default_height.
  This is efficient, and has sufficient expressive power for
  the validation needs. E.g., if only defines_default_height is set,
  that means the width will be auto.

validator.protoascii:

• For all AMP tags, I've looked through the implementations
  in github to see how they override isLayoutSupported. E.g.,
  see here for amp-img.js: builtins/amp-img.js
  isLayoutSizeDefined is a predicate covering fixed,
  fixed-height, responsive, and fill, and in addition,
  nodisplay is always allowed. This, along with the whitelist
  from layout.js for hasNaturalDimensions is what then makes
  up a tag spec.

validator.js is implemented in the same way:

• When AmpLayout is active (as in, the TagSpec under
  consideration has the amp_layout field set), then the four
  layout tags are added as optional via a taglist, $AMP_LAYOUT_ATTRS,
  to disable the usual attribute checks.
• Instead, ValidateLayout is triggered, which works like so:
  • ParseLayout looks up the enum values.
  • CssLengthAndUnit is a parsing routine for the length values
    for width and height that we deal with. I made this into a
    constructor so that I can return four named values. This thing
    supports 'auto', based on a constructor argument.
  • After the input attributes have been parsed (that is, layout,
    width, height), CalculateWidth, CalculateHeight, and
    CalculateLayout compute the effective layout spec of the element.
    This is where the defaults get implemented, and the logic is
    intended to produce the same results as the original.
  • After this, we verify that the computed layout is OK for the
    tag, based on the specification from the protoascii
    (AmpLayout::supported_layouts). In the runtime, this is
    done via the isLayoutSupported hook I mentioned above.
  • After this, we run verification specific to particular layout
    values, e.g., FIXED_HEIGHT requires that the width field is
    set to 'auto'. I kept those errors short in our usual style.

Thanks to @dvoytenko and @Gregable for advice and review
(and catching bugs!).

[Gregable]

LGTM

[powdercloud]

@Gregable and I found some problem unrelated to this change which makes validator/build.py fail, and we'll fix it in another pr. In the meantime I've updated the PR to make Travis happy. Also we should make it so that Travis runs our build.py script to catch problems.