Revert of Remove MessageLoop destruction observer from mojo::Watcher.

mojo/public/cpp/system/watcher.cc

Index: mojo/public/cpp/system/watcher.cc
diff --git a/mojo/public/cpp/system/watcher.cc b/mojo/public/cpp/system/watcher.cc
index fc03c5a0a0e8b1f4e41c01b29705d743e051d34c..d9319fb5bda1fc368378a77e15de03b941783a9e 100644
--- a/mojo/public/cpp/system/watcher.cc
+++ b/mojo/public/cpp/system/watcher.cc
@@ -7,9 +7,44 @@
 #include "base/bind.h"
 #include "base/location.h"
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "mojo/public/c/system/functions.h"
 
 namespace mojo {
+
+class Watcher::MessageLoopObserver
+    : public base::MessageLoop::DestructionObserver {
+ public:
+  explicit MessageLoopObserver(Watcher* watcher) : watcher_(watcher) {
+    base::MessageLoop::current()->AddDestructionObserver(this);
+  }
+
+  ~MessageLoopObserver() override {
+    StopObservingIfNecessary();
+  }
+
+ private:
+  // base::MessageLoop::DestructionObserver:
+  void WillDestroyCurrentMessageLoop() override {
+    StopObservingIfNecessary();
+    if (watcher_->IsWatching()) {
+      // TODO(yzshen): Remove this notification. crbug.com/604762
+      watcher_->OnHandleReady(MOJO_RESULT_ABORTED);
+    }
+  }
+
+  void StopObservingIfNecessary() {
+    if (is_observing_) {
+      is_observing_ = false;
+      base::MessageLoop::current()->RemoveDestructionObserver(this);
+    }
+  }
+
+  bool is_observing_ = true;
+  Watcher* watcher_;
+
+  DISALLOW_COPY_AND_ASSIGN(MessageLoopObserver);
+};
 
 Watcher::Watcher(scoped_refptr<base::SingleThreadTaskRunner> runner)
     : task_runner_(std::move(runner)),
@@ -37,6 +72,7 @@
   DCHECK(!IsWatching());
   DCHECK(!callback.is_null());
 
+  message_loop_observer_.reset(new MessageLoopObserver(this));
   callback_ = callback;
   handle_ = handle;
   MojoResult result = MojoWatch(handle_.value(), signals,
@@ -45,6 +81,7 @@
   if (result != MOJO_RESULT_OK) {
     handle_.set_value(kInvalidHandleValue);
     callback_.Reset();
+    message_loop_observer_.reset();
     DCHECK(result == MOJO_RESULT_FAILED_PRECONDITION ||
            result == MOJO_RESULT_INVALID_ARGUMENT);
     return result;
@@ -62,6 +99,7 @@
 
   MojoResult result =
       MojoCancelWatch(handle_.value(), reinterpret_cast<uintptr_t>(this));
+  message_loop_observer_.reset();
   // |result| may be MOJO_RESULT_INVALID_ARGUMENT if |handle_| has closed, but
   // OnHandleReady has not yet been called.
   DCHECK(result == MOJO_RESULT_INVALID_ARGUMENT || result == MOJO_RESULT_OK);
@@ -74,6 +112,7 @@
 
   ReadyCallback callback = callback_;
   if (result == MOJO_RESULT_CANCELLED) {
+    message_loop_observer_.reset();
     handle_.set_value(kInvalidHandleValue);
     callback_.Reset();
   }