Reapply "Use a single file for app snapshots."

runtime/bin/file_macos.cc

Index: runtime/bin/file_macos.cc
diff --git a/runtime/bin/file_macos.cc b/runtime/bin/file_macos.cc
index 38699cd6d069b63556d5aa89cb167bb597d243ef..0720b6f3437db0d7e218288a3ead4f81d5d9a5de 100644
--- a/runtime/bin/file_macos.cc
+++ b/runtime/bin/file_macos.cc
@@ -80,16 +80,23 @@ bool File::IsClosed() {
 }
 
 
-void* File::MapExecutable(intptr_t* len) {
+void* File::Map(MapType type, int64_t position, int64_t length) {
   ASSERT(handle_->fd() >= 0);
-  intptr_t length = Length();
-  void* addr = mmap(0, length,
-                    PROT_READ | PROT_EXEC, MAP_PRIVATE,
-                    handle_->fd(), 0);
+  int prot = PROT_NONE;
+  switch (type) {
+    case kReadOnly:
+      prot = PROT_READ;
+      break;
+    case kReadExecute:
+      prot = PROT_READ | PROT_EXEC;
+      break;
+    default:
+      return NULL;
+  }
+  void* addr = mmap(NULL, length, prot, MAP_PRIVATE,
+                    handle_->fd(), position);
   if (addr == MAP_FAILED) {
-    *len = -1;
-  } else {
-    *len = length;
+    return NULL;
   }
   return addr;
 }