This patch implements support for the width/height properties on
MathML elements [1]. The general algorithm from the spec is as
follows:

(1) The outcome of the math layout is a "math content box".
(2) The content box sets its size from computed width/height values. If

auto, it's the one of the "math content box". This patch ignores
percentage values for now [2] [3].

(3) math content box is shifted so that its inline-start and top edges

aligns with the ones of the content box. There are exceptions
elements like mfrac and munder/mover/munderover which instead
horizontally center the math content box within the content box.
For baseline adjustment, we follow what Chromium does, see [4].

(4) Padding+border are added around the content box. Note that we

ignore the box-sizing property for now [5].

The patch essentially tweaks the various MathML layout algorithms to
perform steps (3) and (4) before the calls to
GetBorderPaddingForPlace and InflateReflowAndBoundingMetrics.

[1] https://w3c.github.io/mathml-core/#layout-algorithms
[2] https://github.com/w3c/mathml-core/issues/76
[3] https://github.com/w3c/mathml-core/issues/77
[4] https://github.com/w3c/mathml-core/issues/259
[5] https://github.com/w3c/mathml-core/issues/257

Below is more information about test coverage:

• width-height-001: Verify that width, height, inline-size and block-size properties sets the size of the content box. This test used to verify they are ignored, this patch fixes the tag. It also adds a test for the case the specified size is smaller than the content (we force non empty descendants to make sure this content is large enough) and to verify the width is used for the preferred width.

• width-height-002, width-height-003: These are reftests visually checking offsets of the math content box within a larger content box (specified by width/height) for the mtext, mrow, mpadded, mfrac, msqrt, mroot, in LTR/RTL modes. In particular they allow to verify some painted elements like fraction bar and radical symbols.

• width-height-004: This test more directly checks that the math content box is horizontally centered within a larger content box for munder, mover, munderover and mfrac. This patch extends the test to cover the case when the math content box is wider (i.e. overflowing outside the content box) and removes unnecessary specified height.

• width-height-005: New test for other layout algorithm that don't center the math content box, checking inline-start edges of children when a width is specified. We check both LTR/RTL modes and wider/narrower content boxes.

• width-height-006: Same but checking the top edges for larger/smaller height and verifying that baseline is perserved.