Add "Prefer Sequences to Threads" section to Threading and Tasks docs

docs/threading_and_tasks.md

 # Threading and Tasks in Chrome
 
 [TOC]
 
 ## Overview
 
 ### Threads
 
 Every Chrome process has
 
@@ skipping 30 matching lines @@
 
 * [Parallel](#Posting-a-Parallel-Task): No task execution ordering, possibly all
   at once on any thread
 * [Sequenced](#Posting-a-Sequenced-Task): Tasks executed in posting order, one
   at a time on any thread.
 * [Single Threaded](#Posting-Multiple-Tasks-to-the-Same-Thread): Tasks executed
   in posting order, one at a time on a single thread.
    * [COM Single Threaded](#Posting-Tasks-to-a-COM-Single-Thread-Apartment-STA_Thread-Windows_):
      A variant of single threaded with COM initialized.
 
-The discussion below covers all of these ways to execute tasks.
+### Prefer Sequences to Threads
+
+**Sequenced execution mode is far prefered to Single Threaded** in scenarios
+that require mere thread-safety as it opens up scheduling paradigms that
+wouldn't be possible otherwise (sequences can hop threads instead of being stuck
+behind unrelated work on a dedicated thread). Ability to hop threads also means
+the thread count can dynamically adapt to the machine's true resource
+availability (faster on bigger machines, avoids trashing on slower machines).
+
+Many core APIs were recently made sequence-friendly (classes are rarely
+thread-affine -- i.e. only when using thread-local-storage or third-party APIs
+that do). But the codebase has long evolved assuming single-threaded contexts...
+If your class could run on a sequence but is blocked by an overzealous use of
+ThreadChecker/ThreadTaskRunnerHandle/SingleThreadTaskRunner in a leaf
+dependency, consider fixing that dependency for everyone's benefit (or at the
+very least file a blocking bug against https://crbug.com/675631 and flag your
+use of base::CreateSingleThreadTaskRunnerWithTraits() with a TODO against your
+bug to use base::CreateSequencedTaskRunnerWithTraits() when fixed).
+
+The discussion below covers all of these ways to execute tasks in details.
 
 ## Posting a Parallel Task
 
 ### Direct Posting to the Task Scheduler
 
 A task that can run on any thread and doesn’t have ordering or mutual exclusion
 requirements with other tasks should be posted using one of the
 `base::PostTask*()` functions defined in
 [`base/task_scheduler/post_task.h`](https://cs.chromium.org/chromium/src/base/task_scheduler/post_task.h).
 
@@ skipping 555 matching lines @@
 ## TaskRunner ownership
 
 TaskRunners shouldn't be passed through several components. Instead, the
 components that uses a TaskRunner should be the one that creates it.
 
 See [this example](https://codereview.chromium.org/2885173002/) of a
 refactoring where a TaskRunner was passed through a lot of components only to be
 used in an eventual leaf. The leaf can and should now obtain its TaskRunner
 directly from
 [`base/task_scheduler/post_task.h`](https://cs.chromium.org/chromium/src/base/task_scheduler/post_task.h).