Improve line breaking behavior in Firefox and Chrome under complex conditions

Summary: See https://github.com/phacility/phabricator/pull/854. In some situations, line-break: anywhere produces better behavior than word-break: break-all. It never appears to produce worse behavior.

Test Plan:

• Break behavior changes if a line contains "<span />" elements caused by syntax highlighting. This CSS adjustment only appears to apply to text with internal "<span />" elements.
• This specifically impacts certain internal breakpoints adjacent to punctuation, so the test case is highly specific. Generic test cases with latin word characters do not evidence any behavioral changes.
• This change appears to have no impact on Safari, which uses the better behavior in all cases.
• Before Patch: In Firefox and Chrome, this specific change breaks awkwardly. There is more room for text to fit on the broken line:

Firefox

{F7480567}

Chrome

{F7480568}

• After Patch: Firefox and Chrome break the line better. Here's Firefox:

{F7480569}

• Additional context:

Safari Behavior (Unchanged)

{F7480570}

Chrome with no highlighting (desirable behavior). Firefox does the same thing.

{F7480571}

Also tested other cases, which seem never-worse in any browser.

{F7480574}

Differential Revision: https://secure.phabricator.com/D21247