Trust Tokens: Start capturing Element.setAttribute in the TrustTokenIframe use counter

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}

chrome/browser/net/trust_token_usecounter_browsertest.cc

@@ -137,4 +137,34 @@ IN_PROC_BROWSER_TEST_F(TrustTokenUseCountersBrowsertest, CountsIframeUse) {
                                blink::mojom::WebFeature::kTrustTokenIframe, 1);
 }
 
+IN_PROC_BROWSER_TEST_F(TrustTokenUseCountersBrowsertest, CountsIframeUseViaSetattribute) {
+  base::HistogramTester histograms;
+
+  GURL start_url(server_.GetURL("/iframe.html"));
+  EXPECT_TRUE(ui_test_utils::NavigateToURL(browser(), start_url));
+
+  content::WebContents* web_contents =
+      browser()->tab_strip_model()->GetActiveWebContents();
+
+  // It's important to set the trust token arguments before updating src, as
+  // the latter triggers a load. It's also important to JsReplace the trustToken
+  // argument here, because iframe.trustToken expects a (properly escaped)
+  // JSON-encoded string as its value, not a JS object.
+  EXPECT_TRUE(ExecJs(web_contents,
+                     JsReplace(
+                         R"( const myFrame = document.getElementById("test");
+                         myFrame.setAttribute('trustToken', $1);
+                         myFrame.src = $2;)",
+                         R"({"type": "token-request"})", "/page404.html")));
+  TestNavigationObserver load_observer(web_contents);
+  load_observer.Wait();
+
+  // Navigate away in order to flush use counters.
+  EXPECT_TRUE(
+      ui_test_utils::NavigateToURL(browser(), GURL(url::kAboutBlankURL)));
+
+  histograms.ExpectBucketCount("Blink.UseCounter.Features",
+                               blink::mojom::WebFeature::kTrustTokenIframe, 1);
+}
+
 }  // namespace content

third_party/blink/renderer/core/html/html_iframe_element.cc

@@ -247,6 +247,7 @@ void HTMLIFrameElement::ParseAttribute(
       UpdateRequiredPolicy();
     }
   } else if (name == html_names::kTrusttokenAttr) {
+    UseCounter::Count(GetDocument(), WebFeature::kTrustTokenIframe);
     trust_token_ = value;
   } else {
     // Websites picked up a Chromium article that used this non-specified