[WPT/common/security-features] Update spec.src.json for iframe-blank

Follow-up of
https://chromium-review.googlesource.com/c/chromium/src/+/1788876.

The spec validator requires `source_context_schema` to contain
all supported source contexts, so this CL adds entries for
`iframe-blank`, so that generators/validators successfully run.

This CL keeps generated test files unchanged.

Bug: 1001422
Change-Id: Ic49379a57503cd716badeb4655d64e2985e8916e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1889477
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710691}

mixed-content/spec.src.json

@@ -240,6 +240,9 @@
         "http-rp",
         "meta"
       ],
+      "iframe-blank": [
+        "meta"
+      ],
       "srcdoc": [
         "meta"
       ],
@@ -255,6 +258,7 @@
     "supported_subresource": {
       "top": "*",
       "iframe": "*",
+      "iframe-blank": "*",
       "srcdoc": "*",
       "worker-classic": [
         "xhr",

referrer-policy/spec.src.json

@@ -668,6 +668,9 @@
         "meta",
         "http-rp"
       ],
+      "iframe-blank": [
+        "meta"
+      ],
       "srcdoc": [
         "meta"
       ],
@@ -683,6 +686,7 @@
     "supported_subresource": {
       "top": "*",
       "iframe": "*",
+      "iframe-blank": "*",
       "srcdoc": "*",
       "worker-classic": [
         "xhr",