Minor clean-ups to StyleBuilder code

Various small clean-ups to the StyleBuilder:

- Change apply_value_length to a switch statement. I might end up moving
all the generated StyleBuilderFunctions function definitions to
StyleBuilderFunctions later, but I haven't decided how exactly this
should end up looking.

- Use CSSPrimitiveValue->convertToLength instead of manually doing the
conversion for length and border-radius property handlers.
The flag FixedIntegerConversion seems like a misnomer, it calls
computeLength<Length>() for lenghts while FixedFloatConversion calls
Length(computeLength<double>()), the only difference being that the
former will clamp to {min,max}ValueForCssLength. Perhaps we don't need
both options?

- Move marquee-increment handling to StyleBuilderCustom. I put this one
in the wrong place by accident initially.

- Remove extraneous use_none for min-height. I accidentally added this
when moving length properties over. The parser won't allow none so the
generated branch is never taken.

- Rename apply_{value,initial,inherit} to declare_*_function.

BUG=237400

Review URL: https://chromiumcodereview.appspot.com/19326002

git-svn-id: svn://svn.chromium.org/blink/trunk@154294 bbb929c8-8fbe-4397-9dbb-9b2b20218538