Use a single file for app snapshots.

runtime/bin/file_linux.cc

Index: runtime/bin/file_linux.cc
diff --git a/runtime/bin/file_linux.cc b/runtime/bin/file_linux.cc
index 4f61db1d306b3415c629dea7a2f3f10b6564b015..68fce32a882117ecd46251e487470ed6436281c0 100644
--- a/runtime/bin/file_linux.cc
+++ b/runtime/bin/file_linux.cc
@@ -77,16 +77,25 @@ bool File::IsClosed() {
 }
 
 
-void* File::MapExecutable(intptr_t* len) {
+void* File::MapReadOnly(int64_t position, int64_t length) {
+  ASSERT(handle_->fd() >= 0);
+  void* addr = mmap(0, length,
+                    PROT_READ, MAP_PRIVATE,
+                    handle_->fd(), position);
+  if (addr == MAP_FAILED) {
+    return NULL;
+  }
+  return addr;
+}
+
+
+void* File::MapReadExecute(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);
+                    handle_->fd(), position);
   if (addr == MAP_FAILED) {
-    *len = -1;
-  } else {
-    *len = length;
+    return NULL;
   }
   return addr;
 }