base: Add WeakRunner

This is a wrapper around a TaskRunner that allows safely capturing
`this`.

It is going to be used in future commits in TracingServiceImpl. It might
be also nice to use in HeapprofdProducer.

Change-Id: I526128c7f0f91d729b17a7c7c92f1b0e07a52da6
diff --git a/Android.bp b/Android.bp
index 3792d85..12e2e61 100644
--- a/Android.bp
+++ b/Android.bp
@@ -10898,6 +10898,7 @@
         "src/base/virtual_destructors.cc",
         "src/base/waitable_event.cc",
         "src/base/watchdog_posix.cc",
+        "src/base/weak_runner.cc",
     ],
 }