Add new danger type for uncommon or unknown downloads.

chrome/common/extensions/api/experimental.downloads.json

Index: chrome/common/extensions/api/experimental.downloads.json
diff --git a/chrome/common/extensions/api/experimental.downloads.json b/chrome/common/extensions/api/experimental.downloads.json
index 7e2c8c47e95523af28df99732ebe48480eeafee1..ae29981b11bad105d18c48b4abf729fd5e86edfb 100644
--- a/chrome/common/extensions/api/experimental.downloads.json
+++ b/chrome/common/extensions/api/experimental.downloads.json
@@ -68,6 +68,11 @@
         "value": "content",
         "description": "The download's content is malicious."
       },
+      "DANGER_UNCOMMON": {
+        "type": "string",
+        "value": "uncommon",
+        "description": "The download is not commonly downloaded and could be dangerous."
+      },
       "ERROR_GENERIC": {
         "type": "string",
         "value": "I'm afraid I can't do that.",
@@ -104,6 +109,12 @@
         }
       },
       {
+        "id": "DownloadDangerType",
+        "description": "String indicating whether a download is known to be safe or is considered dangerous. The values correspond to the <code>DANGER_SAFE</code>, <code>DANGER_FILE</code>, <code>DANGER_URL</code>, <code>DANGER_CONTENT</code>, and <code>DANGER_UNCOMMON</code> properties.",
+        "type": "string",
+        "enum": ["safe", "file", "url", "content", "uncommon"]
+      },
+      {
         "id": "DownloadItem",
         "type": "object",
         "description": "The state of a file as it is downloaded from the internet.",
@@ -121,9 +132,8 @@
             "description": "absolute local path"
           },
           "danger": {
-            "type": "string",
+            "$ref": "DownloadDangerType",
             "description": "Indication of whether this download is thought to be safe or known to be suspicious.",
-            "enum": ["safe", "file", "url", "content"],
             "optional": true
           },
           "dangerAccepted": {
@@ -285,9 +295,8 @@
             "description": "absolute local path"
           },
           "danger": {
-            "type": "string",
+            "$ref": "DownloadDangerType",
             "description": "Indication of whether this download is thought to be safe or known to be suspicious.",
-            "enum": ["safe", "file", "url"],
             "optional": true
           },
           "dangerAccepted": {