Instead of a single function that takes a base::ExecutionMode as argument,
post_task.h now exposes one task runner creation function per execution
mode. These functions return typed task runners.

  scoped_refptr<TaskRunner> CreateTaskRunnerWithTraits(TaskTraits);
  scoped_refptr<SequencedTaskRunner>
      CreateSequencedTaskRunnerWithTraits(TaskTraits);
  scoped_refptr<SingleThreadTaskRunner>
      CreateSingleThreadTaskRunnerWithTraits(TaskTraits);

We had originally explicitly opted against providing typed TaskRunners per them
lacking all other traits and thus merely being weakly typed (a compile time name
for every combination of trait would be ridiculous). It is however now becoming
obvious that the ExecutionMode is by far the most important type to spread
through the codebase as having a compile time guarantee that something is
single-threaded or sequenced is a pretty big deal and such usage is spread
throughout the codebase already. Other traits (e.g. file I/O allowed) are also
important but more easily enforced (e.g. AssertIOAllowed()) and we are thus fine
keeping the compile-time enforcement to the execution mode only.

This, in general, doesn't change the desire to have components get their own
TaskRunner instead of having it passed in in the TaskScheduler world.

BUG=553459

Review-Url: https://codereview.chromium.org/2464963002
Cr-Commit-Position: refs/heads/master@{#429276}