Bug 1448374 - Loading a .javascript file from a WebExtension's web_accessible_resources messing with macOS file associations r=jimm

From content processes, on Mac, use ContentHandlerService::GetTypeFromExtension()
instead of trying to query the OS directly for MIME information.
Trying to get MIME information from the OS is blocked by content process
sandboxing on Mac.

MozReview-Commit-ID: KGJHDBklxvb

--HG--
extra : rebase_source : ae46525eee622d64ffc6e263b19682aec033480a

uriloader/exthandler/mac/nsOSHelperAppService.h

@@ -32,6 +32,10 @@ public:
                                           bool *found,
                                           nsIHandlerInfo **_retval) override;
 
+  // override so we can have a child process sandbox-friendly implementation
+  bool GetMIMETypeFromOSForExtension(const nsACString& aExtension,
+                                     nsACString& aMIMEType) override;
+
   // GetFileTokenForPath must be implemented by each platform. 
   // platformAppPath --> a platform specific path to an application that we got out of the 
   //                     rdf data source. This can be a mac file spec, a unix path or a windows path depending on the platform

uriloader/exthandler/mac/nsOSHelperAppService.mm

@@ -14,6 +14,7 @@
 #include "nsTArray.h"
 #include "nsIURL.h"
 #include "nsIFile.h"
+#include "nsIHandlerService.h"
 #include "nsILocalFileMac.h"
 #include "nsMimeTypes.h"
 #include "nsIStringBundle.h"
@@ -573,3 +574,34 @@ nsOSHelperAppService::GetProtocolHandlerInfoFromOS(const nsACString &aScheme,
   return NS_OK;
 }
 
+/*
+ * Override GetMIMETypeFromOSForExtension() so that we can proxy requests for
+ * the MIME type to the parent when we're executing in the child process. If
+ * we're in the parent process, query the OS directly.
+ */
+bool
+nsOSHelperAppService::GetMIMETypeFromOSForExtension(const nsACString& aExtension,
+                                                    nsACString& aMIMEType)
+{
+  if (XRE_IsParentProcess()) {
+    return nsExternalHelperAppService::GetMIMETypeFromOSForExtension(aExtension,
+                                                                     aMIMEType);
+  }
+
+  nsCOMPtr<nsIHandlerService> handlerSvc =
+    do_GetService(NS_HANDLERSERVICE_CONTRACTID);
+  if (NS_WARN_IF(!handlerSvc)) {
+    return false;
+  }
+
+  nsresult rv = handlerSvc->GetTypeFromExtension(aExtension, aMIMEType);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return false;
+  }
+
+  if (aMIMEType.IsEmpty()) {
+    return false;
+  }
+
+  return true;
+}