image_decoder has been refactored into a Leaky (accessible from multiple threads) LazyInstance (singleton), that uses one utility process running in batch mode to decode all images.

This CL fixes a performance problem with scrolling in bookmarks on Android.
Previously we were using a new image_decoder and therefore a
new utility process for each image we wanted to decode. Now we
use one shared image_decoder that has one utility process running in batch mode for decoding all images.

BUG=451201

Review URL: https://codereview.chromium.org/931993002

Cr-Commit-Position: refs/heads/master@{#322383}