Extract some logic from transport_dib into shared_memory.

base/memory/shared_memory_posix.cc

Index: base/memory/shared_memory_posix.cc
diff --git a/base/memory/shared_memory_posix.cc b/base/memory/shared_memory_posix.cc
index c80032e05ba73dd5d9c5a6781aeb287a6da00fd6..94b3ee98f46dc4b2269f6451d66cbcfa5e0411ea 100644
--- a/base/memory/shared_memory_posix.cc
+++ b/base/memory/shared_memory_posix.cc
@@ -205,6 +205,15 @@ bool SharedMemory::CreateAndMapAnonymous(size_t size) {
 }
 
 #if !defined(OS_ANDROID)
+// static
+int SharedMemory::GetSizeFromSharedMemoryHandle(
+    const SharedMemoryHandle& handle) {
+  struct stat st;
+  if (fstat(handle.fd, &st) != 0)
+    return st.st_size;
+  return -1;
+}
+
 // Chromium mostly only uses the unique/private shmem as specified by
 // "name == L"". The exception is in the StatsTable.
 // TODO(jrg): there is no way to "clean up" all unused named shmem if