servo: Merge #18365 - stylo: Store location information for keyframe rules (from chenpighead:stylo-keyframe-location); r=upsuper,emilio

So far, we only store location info for the whole Keyframes block, not for each
of the keyframe rule. Without this info, the devtool can't present the rules
on the devtool panel properly.

In this patch, we collect the source location info while parsing keyframe selector.
The binding function, Servo_KeyframesRule_GetKeyframe, is also fixed (and renamed
to Servo_KeyframesRule_GetKeyframeAt to match the fix) to accept line and column
parameters from Gecko, so we can pass/set them with the source location from Servo.

This is the servo part of [Bug 1394994](https://bugzilla.mozilla.org/show_bug.cgi?id=1394994).

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [Bug 1394994](https://bugzilla.mozilla.org/show_bug.cgi?id=1394994)

Source-Repo: https://github.com/servo/servo
Source-Revision: f648e12935cb94405a815216391f2527912ba4c2

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 17bc4073e4d17b26e9d8c07a8ed4b996cfe1612f