Web Inspector: popovers for function source code are malformed

Web Inspector: popovers for function source code are malformed
https://bugs.webkit.org/show_bug.cgi?id=190859

Reviewed by Joseph Pecoraro.

Previously, the `WI.Popover` would be drawn twice: once when the
`WI.CodeMirrorTokenTrackingController` determines that the user has hovered on a function
token, and once again when the source for that function is formatted (after being retrieved).
In the case that the formatter is able to return the prettified source within a frame (or
two), the `WI.Popover` is still in the middle of animating to its new size, meaning that the
changes made by the formatted `update` will be overridden on the next rAF (creates a flash).

* UserInterface/Views/Popover.js:
(WI.Popover.prototype):
(WI.Popover.prototype._animateFrame.drawBackground):
(WI.Popover.prototype._drawBackground):
Add a member variable to make sure that there is only ever one rAF firing at a time.
Drive-by: rework the background code to only use one canvas.
* UserInterface/Views/SourceCodeTextEditor.js:
(WI.SourceCodeTextEditor.prototype._showPopoverForFunction.didGetDetails):
Don't show the `WI.Popover` until the formatter has finished prettifying the function's
source code, as otherwise there is brief moment that the popover appears and is empty.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@237401 268f45cc-cd09-0410-ab3c-d52691b4dbfc