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Implements HoldingSpaceThumbnailLoader - a helper class that uses
image loader extension to generate thumbnail bitmaps for file system
file paths. The class establishes a native connection with the image
loader extension, and sends it a message with the requested image
parameter. The message format should match the image loader request.

Upon response from the image loader, the HoldingSpaceThumbnailLoader
closes the connection to the extension, converts the data received
from the extension into a bitmap, and returns the bitmap to the caller.

On image loader side, implements onConnectNative listener to handle
image requests.

BUG=1113772

Change-Id: Icff6ead726b3be8945ab58b7ff177259707ebc45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2362973
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#802717}