introduce a background-hang-monitor:

Mac-Os implementation of a thread sampler,
Linux and Windows skeleton implementations.

components/background_hang_monitor/sampler_linux.rs

@@ -0,0 +1,35 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use crate::sampler::{NativeStack, Sampler};
+use libc;
+
+type MonitoredThreadId = libc::pthread_t;
+
+#[allow(dead_code)]
+pub struct LinuxSampler {
+    thread_id: MonitoredThreadId,
+}
+
+impl LinuxSampler {
+    #[allow(unsafe_code, dead_code)]
+    pub fn new() -> Box<Sampler> {
+        let thread_id = unsafe { libc::pthread_self() };
+        Box::new(LinuxSampler { thread_id })
+    }
+}
+
+impl Sampler for LinuxSampler {
+    #[allow(unsafe_code)]
+    fn suspend_and_sample_thread(&self) -> Result<NativeStack, ()> {
+        // Warning: The "critical section" begins here.
+        // In the critical section:
+        // we must not do any dynamic memory allocation,
+        // nor try to acquire any lock
+        // or any other unshareable resource.
+
+        // NOTE: End of "critical section".
+        Err(())
+    }
+}