Bug 985304 part 0: Drop "running" prefix on variable 'runningFlexWeightSum', for clarity. r=mats
Bug 985304 part 1: Shorten "flex weight" to "weight" in variable & function names. r=mats

The phrase "flex weight" used to be a term defined in the spec, but it's
long-since been replaced with "flex factor" and "scaled flex factor".

I'm now using the term "weight" to mean "flex factor, with any necessary scaling already done, ready to be used as a weight for apportioning free space".

I'm dropping the "flex" modifier from "flex weight" both for brevity & to make
it more clearly-distinguishable from actual "flex factor" usages in the code.
Bug 985304 part 2: Make flex algorithm gracefully handle flex factors that sum to < 1. r=mats

When our flexibilities sum to a fraction < 1, we clamp the free space to be no
greater than that same fraction of the original free space. This effectively
makes us treat a flexibility of e.g. "0.01" as "1% of the original free space"
in that sort of situation (while still treating it as a normal flex factor if
the sum of the flex factors is greater than 1).
Bug 985304 part 3: Extend flexbox layout mochitest to cover cases with flex values that sum to less than 1. f=mats