Add atan() function for CSSTrigonometrcFunctions

[chromium-wpt-export-bot]

According to the CSS Values and Units Module Level 4 [1], the trigonometric function should be parsed and evaluated.

This CL adds atan() function and testcases for atan().

Add conditionally evaluate inf or -inf statement for vertical asymptote value from tan().

Feature status: [2] Intent to prototype: [3]

[1] https://www.w3.org/TR/css-values-4/#trig-funcs [2] https://chromestatus.com/feature/5165381072191488 [3] https://groups.google.com/a/chromium.org/g/blink-dev/c/-c9p-Sq_gWg

Bug: 1190444 Change-Id: I55b4f92008c42bd13d7f9a68c562e5f5b894c8c9 Reviewed-on: https://chromium-review.googlesource.com/3893915 WPT-Export-Revision: c1db634abfc24069e882a0dd50fb7337e0936140