refactor(browser): Make bfcache frame purely positional

`frame` no longer special-cases the `masked` reason value; it now reflects
only the frame's position in the notRestoredReasons tree (`top`/`child`).
The `masked` value already lives on the `reason` attribute, so tagging the
frame as `masked` too was redundant. The integration no longer inspects any
reason string - reasons pass through verbatim.

Author: logaretm

dev-packages/e2e-tests/test-applications/browser-bfcache/tests/bfcache.test.ts

@@ -47,8 +47,8 @@ test('reports a miss with notRestoredReasons when an unload listener blocks bfca
     metric =>
       metric.name === 'browser.bfcache.not_restored' && attr(metric, 'browser.bfcache.reason') === 'unload-listener',
   );
-  // Chrome reports a privacy-masked reason alongside the real one; the integration must classify it
-  // as a `masked` frame. This is our only real-browser coverage of the masked-frame path.
+  // Chrome reports a privacy-masked reason alongside the real one. It's a top-frame reason here, so
+  // the integration must frame it positionally (`top`) and pass the value through untouched.
   const maskedReasonPromise = waitForMetric(
     PROXY_SERVER_NAME,
     metric => metric.name === 'browser.bfcache.not_restored' && attr(metric, 'browser.bfcache.reason') === 'masked',
@@ -82,7 +82,7 @@ test('reports a miss with notRestoredReasons when an unload listener blocks bfca
   expect(attr(unloadReason, 'browser.bfcache.frame')).toBe('top');
 
   const maskedReason = await maskedReasonPromise;
-  expect(attr(maskedReason, 'browser.bfcache.frame')).toBe('masked');
+  expect(attr(maskedReason, 'browser.bfcache.frame')).toBe('top');
 
   const reloadDuration = await reloadDurationPromise;
   expect(reloadDuration.type).toBe('distribution');

packages/browser/src/integrations/bfcache.ts

@@ -6,7 +6,7 @@ import { WINDOW } from '../helpers';
 const INTEGRATION_NAME = 'BFCacheMetrics';
 const DEFAULT_MAX_REASONS = 5;
 
-type BFCacheFrame = 'top' | 'child' | 'masked' | 'unknown';
+type BFCacheFrame = 'top' | 'child';
 
 interface BFCacheIntegrationOptions {
   /**
@@ -158,7 +158,7 @@ function _collectReasonsFromFrame(
 
     collectedReasons.push({
       reason: reasonValue,
-      frame: reasonValue === 'masked' ? 'masked' : frameType,
+      frame: frameType,
     });
   });
 

packages/browser/test/integrations/bfcache.test.ts

@@ -71,15 +71,15 @@ describe('bfcacheMetricsIntegration', () => {
       expect(_collectNotRestoredReasons(tree, 5)).toEqual([{ reason: 'fetch', frame: 'child' }]);
     });
 
-    it('marks the "masked" reason as a masked frame regardless of depth', () => {
+    it('frames every reason by its position, without special-casing the reason value', () => {
       const tree = {
         reasons: [{ reason: 'masked' }],
         children: [{ reasons: [{ reason: 'masked' }] }],
       };
 
       expect(_collectNotRestoredReasons(tree, 5)).toEqual([
-        { reason: 'masked', frame: 'masked' },
-        { reason: 'masked', frame: 'masked' },
+        { reason: 'masked', frame: 'top' },
+        { reason: 'masked', frame: 'child' },
       ]);
     });