[Frame Details] Remove unnecessary 'instanceof' check
[Frame Details] Remove unnecessary 'instanceof' check

In the new FrameDetailsView.ts component, the `this.frame` always is a
`ResourceTreeFrame`. This check was useful earlier when the same
function was called by `OpenedWindowDetailsView` as well.

Bug: 1165710
Change-Id: I9ab6b779e7fc43928a2d94b896f1eeb966272470
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2678090
Reviewed-by: Sigurd Schneider <f04d242e109628a27a0f75fd49a87722bcf291fd@chromium.org>
Commit-Queue: Wolfgang Beyer <573aa99bb19da47d816981118f95c945310dd3f4@chromium.org>

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend