Introduce JPEG thumbnails for Grid Tab Switcher

The thumbnails shown in the Grid Tab Switcher (GTS) were decoded from
the ETC1 format, downsampled, and then transferred through the JNI
boundary. This is not the most efficient way.

This CL introduces JPEG thumbnails, which would be saved at the same
time ETC1 thumbnails are saved, if GTS is enabled. The GTS would
directly read and decode the JPEG files from the Java side.

Bug: 971939
Change-Id: Iaf21ea2170afbb7b0385570459f4566715786ca0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648963
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667302}