Refactor use of actual values in computePositionedLogicalWidthUsing
Refactor use of actual values in computePositionedLogicalWidthUsing

By hosting the actual values for logical left and right at the
beginning of the function (that needs to be resolved anyway),
we can simplify the function's logic: now availableSpace is
really the available space as it includes any specified logical
left / right. Also left and right now behave symmetrically.

Review URL: https://codereview.chromium.org/761663002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@186048 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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