Allow embedding Perfetto UI in Tensorboard pages (#7048)

## Motivation for features / changes
This is a required step for open sourcing the new Megascale Perfetto
tool in xprof.

Googlers see b/476134328

## Technical description of changes
Adds https://ui.perfetto.dev to the whitelist of domains that are
allowed to be embedded in a frame on the page. A follow up change will
embed this UI in the xprof (Profiler) plugin.

## Screenshots of UI changes (or N/A)
N/A

## Detailed steps to verify changes work correctly (as executed by you)
Tested on Tensorboard (in combination with other necessary changes) and
verified that it works as expected.

## Alternate designs / implementations considered (or N/A)
N/A

Author: bbadawi1

tensorboard/backend/http_util.py

@@ -36,7 +36,7 @@
 # @vaadin/vaadin-lumo-styles/font-icons(via vaadin-grid) uses data URI for
 # loading font icons.
 _CSP_FONT_DOMAINS_WHITELIST = ["data:"]
-_CSP_FRAME_DOMAINS_WHITELIST = []
+_CSP_FRAME_DOMAINS_WHITELIST = ["https://ui.perfetto.dev"]
 _CSP_IMG_DOMAINS_WHITELIST = []
 _CSP_SCRIPT_DOMAINS_WHITELIST = []
 _CSP_CONNECT_DOMAINS_WHITELIST = []