DevTools: Improper function call in Styles pane
DevTools: Improper function call in Styles pane

A bug in the Styles pane exists due to un-followed typing within
the _setActiveProperty function. The line previous to the expression
in question goes through treeElement.property, which is a
StylePropertyTreeElement. Its property `ownerStyle` returns an
SDK.CSSStyleDeclaration. We can see that SDK.CSSStyleDeclaration is
initialized with a property `parentRule` of type SDK.CSSRule, which
is the inferred type of `rule` in the outcome of the line in question.
However, SDK.CSSRule does not contain a member function called
`selectorText()`; only SDK.CSSStyleRule does.

Ostensibly it's possible that this is of type CSSKeyframesRule or
CSSKeyframeRule, and neither of these contains meaningful definitions
of something that matches a "selector," so it isn't really practically
possible to address via polymorphism.

This corrects the value-check and only calls the function if the
rule in question is of type SDK.CSSStyleRule.

Change-Id: I960e2b49ce9734134b55007149a39a97861739dd
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1872447
Reviewed-by: Lorne Mitchell <b024a4e2ee56b9b5450225fa7affa8f95408a018@microsoft.com>
Commit-Queue: Lorne Mitchell <b024a4e2ee56b9b5450225fa7affa8f95408a018@microsoft.com>

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