Bug 1677000 - Address unpublished clang-tidy lints r=Gijs

There are two new lints introduced since IOUtils was written that we're hitting
now:

* IOUtils::InternalFileInfo's constructor does not initialize `mType`, `mSize`,
  `mLastModified`, and `mPermissions`; and
* We should be using a nested namespace statement.

We haven't hit them since these lines haven't been touched, but I noticed them
on the code review frontend.

Differential Revision: https://phabricator.services.mozilla.com/D99163

dom/system/IOUtils.cpp

@@ -59,8 +59,7 @@
     }                                                                    \
   } while (0)
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 // static helper functions
 
@@ -1417,8 +1416,7 @@ IOUtils::InternalWriteOpts::FromBinding(const WriteOptions& aOptions) {
   return opts;
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #undef REJECT_IF_SHUTTING_DOWN
 #undef REJECT_IF_INIT_PATH_FAILED

dom/system/IOUtils.h

@@ -407,11 +407,11 @@ class IOUtils::IOError {
  */
 struct IOUtils::InternalFileInfo {
   nsString mPath;
-  FileType mType;
-  uint64_t mSize;
-  uint64_t mLastModified;
+  FileType mType = FileType::Other;
+  uint64_t mSize = 0;
+  uint64_t mLastModified = 0;
   Maybe<uint64_t> mCreationTime;
-  uint32_t mPermissions;
+  uint32_t mPermissions = 0;
 };
 
 /**