CSS Calc: Use Lengths instead of numbers to represent pixels

Platform CalcExpressionNumber is overloaded with two uses semantic
meanings: numbers and pixel lengths. This makes conversion from platform
calcs to CSS calcs impossible to do accurately.

BUG=260097

Review URL: https://codereview.chromium.org/23439004

git-svn-id: svn://svn.chromium.org/blink/trunk@158790 bbb929c8-8fbe-4397-9dbb-9b2b20218538
De-assign UseCounter sample IDs for -internal-* CSS properties.

As UseCounter does not count the usage of internal CSS properties, it makes no
sense to keep the sample IDs (IDs used in histograms) for these properties.

This refactoring intends to prevent unnecessary and unintended addition to
UseCounter sample IDs in the future.

This CL is a simple refactoring and does not intend to change any behavior.

BUG=none
R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/25216002

git-svn-id: svn://svn.chromium.org/blink/trunk@158791 bbb929c8-8fbe-4397-9dbb-9b2b20218538