Get media::learning::LearningTasks by name rather than ID.

The primary purpose of a LearningTask is to describe/configure a
LearningTaskController. LearningTaskController's are registered
by name, whereas LearningTasks have been registered by enum ID. This CL
remedies the asymmetry, making retrieval of Tasks match that of
Controllers by using a "name" in both places.

Change-Id: Ibc58b42e9c92876134f3da524b0804f3e1d71809
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2053619
Commit-Queue: Frank Liberato <liberato@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Auto-Submit: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741100}

• media/blink/webmediaplayer_impl.cc[diff]
• media/blink/webmediaplayer_impl.h[diff]
• media/learning/common/media_learning_tasks.cc[diff]
• media/learning/common/media_learning_tasks.h[diff]
• media/learning/common/media_learning_tasks_unittest.cc[diff]