Pass file Regions along with FDs to child processes on Android

base/files/memory_mapped_file.cc

Index: base/files/memory_mapped_file.cc
diff --git a/base/files/memory_mapped_file.cc b/base/files/memory_mapped_file.cc
index 891fcffc4cb434b74729657b7981a400729f2e61..cfb19d21232d0a35b93254b1b44cdf981e2ec3bb 100644
--- a/base/files/memory_mapped_file.cc
+++ b/base/files/memory_mapped_file.cc
@@ -16,10 +16,11 @@ const MemoryMappedFile::Region MemoryMappedFile::Region::kWholeFile(
 MemoryMappedFile::Region::Region(base::LinkerInitialized) : offset(0), size(0) {
 }
 
+MemoryMappedFile::Region::Region() : offset(-1), size(-1) {
+}

[Primiano Tucci (use gerrit), 2015/05/29 10:44:34]

Not sure I get what is the use case for the empty ctor here. Is it different
from kWholeFile? Why kWholeFile is (0,0) and the default ctor is (-1,-1).
If this is just to get this into a map, isn't just a matter of having explicit
copy and assignment ctors?

[agrieve, 2015/05/29 19:16:39]

There's two parts to this:
1. Why have a default constructor at all?
- I think we could continue to get away without one, but since this is a struct
that is now being used as an out parameter, it's quite annoying for it to not
have a default constructor. Unless there's a compelling reason to not have one,
I think it should.

2. What should the default values be:
-There's two decent options here: kWholeFile, or -1, -1 (invalid values). 
-I think either are reasonable, and I don't see a strong case for one over the
other (although I do slightly favour invalid values).

[Primiano Tucci (use gerrit), 2015/06/01 09:14:53]

Oh, I see the point, makes sense.
SGTM.

+
 MemoryMappedFile::Region::Region(int64 offset, int64 size)
     : offset(offset), size(size) {
-  DCHECK_GE(offset, 0);
-  DCHECK_GT(size, 0);
 }
 
 bool MemoryMappedFile::Region::operator==(