simplify posix MappedFile::Init and add useful logs

This ancient CL suggested a simplication in mapped_file_posix.cc:
https://codereview.chromium.org/160288

After hitting this DCHECK on an embedded device where using a debugger
is difficult, I decided to make the suggested change and add some useful
logging while I was at it.

BUG=391937

Review URL: https://codereview.chromium.org/372113003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@281747 0039d316-1c4b-4281-b951-d872f2087c98

net/disk_cache/blockfile/mapped_file_posix.cc

@@ -26,8 +26,8 @@ void* MappedFile::Init(const base::FilePath& name, size_t size) {
                  platform_file(), 0);
   init_ = true;
   view_size_ = size;
-  DCHECK(reinterpret_cast<intptr_t>(buffer_) != -1);
-  if (reinterpret_cast<intptr_t>(buffer_) == -1)
+  DPLOG_IF(FATAL, buffer_ == MAP_FAILED) << "Failed to mmap " << name.value();
+  if (buffer_ == MAP_FAILED)
     buffer_ = 0;
 
   // Make sure we detect hardware failures reading the headers.