Revert of Revert of Mojo: Convert scoped_ptr<...>() to nullptr in...

Revert of Revert of Mojo: Convert scoped_ptr<...>() to nullptr in mojo/embedder. (patchset #1 id:1 of https://codereview.chromium.org/593313006/)

Reason for revert:
scoped_ptr nullptr support was un-reverted.

Original issue's description:
> Revert of Mojo: Convert scoped_ptr<...>() to nullptr in mojo/embedder. (patchset #1 id:1 of https://codereview.chromium.org/606143003/)
>
> Reason for revert:
> scoped_ptr nullptr support was reverted.
>
> Original issue's description:
> > Mojo: Convert scoped_ptr<...>() to nullptr in mojo/embedder.
> >
> > R=jamesr@chromium.org
> >
> > Committed: https://crrev.com/1b198fa11edbe68701e12c27d5f4981398f5ca71
> > Cr-Commit-Position: refs/heads/master@{#297093}
>
> TBR=jamesr@chromium.org,viettrungluu@chromium.org
> NOTREECHECKS=true
> NOTRY=true
>
> Committed: https://crrev.com/76577a68922585f5621db8c63ba2ab6d5aa03f69
> Cr-Commit-Position: refs/heads/master@{#297109}

TBR=jamesr@chromium.org,dcheng@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/612923002

Cr-Commit-Position: refs/heads/master@{#297191}

mojo/embedder/simple_platform_shared_buffer.cc

@@ -52,7 +52,7 @@ scoped_ptr<PlatformSharedBufferMapping> SimplePlatformSharedBuffer::Map(
     size_t offset,
     size_t length) {
   if (!IsValidMap(offset, length))
-    return scoped_ptr<PlatformSharedBufferMapping>();
+    return nullptr;
 
   return MapNoCheck(offset, length);
 }

mojo/embedder/simple_platform_shared_buffer_posix.cc

@@ -137,7 +137,7 @@ scoped_ptr<PlatformSharedBufferMapping> SimplePlatformSharedBuffer::MapImpl(
   // return null either.
   if (real_base == MAP_FAILED || !real_base) {
     PLOG(ERROR) << "mmap";
-    return scoped_ptr<PlatformSharedBufferMapping>();
+    return nullptr;
   }
 
   void* base = static_cast<char*>(real_base) + offset_rounding;

mojo/embedder/simple_platform_shared_buffer_win.cc

@@ -75,7 +75,7 @@ scoped_ptr<PlatformSharedBufferMapping> SimplePlatformSharedBuffer::MapImpl(
                                   real_length);
   if (!real_base) {
     PLOG(ERROR) << "MapViewOfFile";
-    return scoped_ptr<PlatformSharedBufferMapping>();
+    return nullptr;
   }
 
   void* base = static_cast<char*>(real_base) + offset_rounding;