Bug 1872997 - Keep clipping tooltip container overflow to fix a mochitest.

browser_html_tooltip_variable-height.js has a 300px tall tooltip, along
with 600px tall content, which overflows.

The click outside of the tooltip goes to the content unless it's
clipped... I'm not sure that's the intention of the test but it seems to
match the pre-existing behavior.

MANUAL PUSH: Trivial orange fix CLOSED TREE.

devtools/client/themes/tooltips.css

@@ -215,6 +215,7 @@ strong {
   z-index: 9999;
   background: transparent;
   pointer-events: none;
+  overflow: clip;
   filter: drop-shadow(0 2px 8px var(--theme-tooltip-shadow-color));
 }