Cleanup TaskObserver references

Users should include base/task/task_observer.h and use the real thing
instead of using any of the many typedefs.

Had to fix a lot of includes as message_loop.h no longer includes
message_loop_current.h which in turn includes a lot of other stuff
files were relying on.

This is a mechanical change that will be reviewed according to
https://chromium.googlesource.com/chromium/src/+/master/docs/code_reviews.md#mechanical-changes

Bug: 891670

[email protected]

Change-Id: I101ae66150e18f992da1043ced284b41e4addff8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1738552
Commit-Queue: Carlos Caballero <[email protected]>
Reviewed-by: Gabriel Charette <[email protected]>
Cr-Commit-Position: refs/heads/master@{#685980}
46 files changed