[Paint Preview] Add documentation for player

Change-Id: I7e203ddd04ab61e1f480db09380ec90d7e95ffc7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2583002
Commit-Queue: Mehran Mahmoudi <mahmoudi@chromium.org>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836197}

components/paint_preview/player/README.md

+# Paint Preview Player
+
+The player displays a paint preview that has been previously recorded.
+Currently, the player is only fully implemented for Android. However, there
+are a few platform-independent base classes ([`PlayerCompositorDelegate`](https://source.chromium.org/chromium/chromium/src/+/master:/components/paint_preview/player/player_compositor_delegate.cc)
+, [`CompositorStatus`](https://source.chromium.org/chromium/chromium/src/+/master:/components/paint_preview/player/compositor_status.h)
+, [`BitmapRequest `](https://source.chromium.org/chromium/chromium/src/+/master:/components/paint_preview/player/bitmap_request.cc)
+) than can be used to extend the playback support for other platforms.
+
+`PlayerCompositorDelegate` uses the [StartCompositorService](https://source.chromium.org/chromium/chromium/src/+/master:components/paint_preview/browser/compositor_utils.h;bpv=1;bpt=1;l=16?q=StartCompositorService&ss=chromium&gsn=StartCompositorService&gs=kythe%3A%2F%2Fchromium.googlesource.com%2Fchromium%2Fsrc%3Flang%3Dc%252B%252B%3Fpath%3Dsrc%2Fcomponents%2Fpaint_preview%2Fbrowser%2Fcompositor_utils.h%23E_S97S1y6_-ukZj8vR6XZUzOTYFPwOkwR0LybvqxVWg)
+API. Alternatively, playback support for other platforms can be provided by
+using `StartCompositorService` directly for more flexibility.
+
+The remainder of this doc describes the Android-specific implementation.
+
+## TL;DR
+
+Want to use the player? Construct a [`PlayerManager`](https://source.chromium.org/chromium/chromium/src/+/master:/components/paint_preview/player/android/java/src/org/chromium/components/paintpreview/player/PlayerManager.java)
+and use [`PlayerManager#getView`](https://source.chromium.org/chromium/chromium/src/+/master:components/paint_preview/player/android/java/src/org/chromium/components/paintpreview/player/PlayerManager.java;drc=c62b2799aeef847ccb9fbd5d1fc3e19e2938df9a;l=240)
+to display it.
+
+## Design
+
+As mentioned in the main [`README`](https://source.chromium.org/chromium/chromium/src/+/master:/components/paint_preview/README.md)
+, a paint preview can have multiple frames in a nested structure.
+Consequently, the player is desinged in a nested structure as well, to
+facilitate the display of mulitple nested frames.
+
+
+* `android/java/src/.../player/`: This directory contains per-player classes.
+In another word, these classes are aware that the player might have multiple
+frames and are not involved in the logic for displaying a single frame.
+* `android/java/src/.../player/frame`: This directory contains per-frame
+classes. These are responsible for displaying a single frame.
+
+### Important classes
+
+* [`PlayerManager`](https://source.chromium.org/chromium/chromium/src/+/master:/components/paint_preview/player/android/java/src/org/chromium/components/paintpreview/player/PlayerManager.java):
+Entry point for using the player. When created it initializes the compositor
+and populates a hierarchy of player frames based on the paint preview.
+* [`PlayerCompositorDelegateImpl`](https://source.chromium.org/chromium/chromium/src/+/master:/components/paint_preview/player/android/java/src/org/chromium/components/paintpreview/player/PlayerCompositorDelegateImpl.java)
+: Communicates with the paint preview compositor. It requests bitmaps from the
+comopsitor and provides them to palyer frames.
+* [`PlayerFrameMediator`](https://source.chromium.org/chromium/chromium/src/+/master:/components/paint_preview/player/android/java/src/org/chromium/components/paintpreview/player/frame/PlayerFrameMediator.java)
+: Handles the business logic for a single frame in the player, i.e.
+maintaining a viewport, updating its sub-frame visibilities, requesting
+bitmaps from the compositor delegate, etc.
+* [`PlayerFrameView`](https://source.chromium.org/chromium/chromium/src/+/master:/components/paint_preview/player/android/java/src/org/chromium/components/paintpreview/player/frame/PlayerFrameView.java)
+: The View that displays a single frame.
\ No newline at end of file