Adding helper functions to create and check for Length variants

Instead of Length(123.45, kFixed) -> Length::Fixed(123.45)
Instead of Length(kAuto) -> Length::Auto()
...

I have a plan to make kAuto and kFixed non-blink-globals (by making
their enum an enum class) but that will make code using those
constants more verbose so to reduce the effect, this patch
greatly reduces the use of those constants by wrapping them in helper
functions.

I believe this also makes the code a little bit more readable but
since it is different it will mean learning something new.

The old way still works.

Change-Id: I1e88be433dc5819e62832522ec90e48a8409ebc3
Reviewed-on: https://chromium-review.googlesource.com/c/1460919
Commit-Queue: Daniel Bratell <bratell@opera.com>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631634}
75 files changed