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The TrustTokenIframe use counter is intended to roughly trigger whenever a page will load an iframe that has its 'trusttoken' attribute set. Currently, this doesn't capture calls to Element.setAttribute: see the discussion on blink-reviews-bindings at https://groups.google.com/a/chromium.org/g/blink-reviews-bindings/c/HAEfgM7JkDk/m/JUAZuxI1AQAJ. This makes it undercount use of the feature in the wild. This change adds an additional UseCounter::Count call in html_iframe_element.cc in order to cover this case. 

Change-Id: Id4830632cdc234962762bf601fc902deb6f7ab0b
Test: Adds a browser test covering this case
Bug: 1072163
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2616246Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Auto-Submit: David Van Cleve <davidvc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841640}