Avoid double = and assign nullptr instead of 0

This is in preparation for transition to CheckedPtr (see go/miracleptr).
Assigning 0 to CheckedPtr is ambiguous.
Double = can be a problem, if only one of the fields gets converted to
CheckedPtr.

Bug: 1080832
Change-Id: I0ebc75e89b8d2ea0e7d63f4c0dcbcaab8aecf7b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2434022Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811499}

net/disk_cache/blockfile/mapped_file_avoid_mmap_posix.cc

@@ -14,7 +14,7 @@ namespace disk_cache {
 void* MappedFile::Init(const base::FilePath& name, size_t size) {
   DCHECK(!init_);
   if (init_ || !File::Init(name))
-    return NULL;
+    return nullptr;
 
   if (!size)
     size = GetLength();
@@ -26,7 +26,8 @@ void* MappedFile::Init(const base::FilePath& name, size_t size) {
   } else {
     free(buffer_);
     free(snapshot_);
-    buffer_ = snapshot_ = 0;
+    buffer_ = nullptr;
+    snapshot_ = nullptr;
   }
 
   init_ = true;