khushalsagar@, we build the |sync_decoded_images| vector https://cs.chromium.org/chromium/src/cc/tiles/tile_manager.cc?dr&l=908 based on |images_in_tile| which is discardable imgs in tiles' enclosing layer rect.

Do you mean we should get the discardable imgs from tiles' invalidated content rect instead i.e. enclosing layer rect of invalidated content rect ?

Yup. The change should be to query images only from the invalidated rect. But whether that is used will depend on if the a resource could be acquired to do partial raster. That happens in CreateRasterTask (https://cs.chromium.org/chromium/src/cc/tiles/tile_manager.cc?dr&l=1112), which also sets the invalidated_rect that will be rasterized in the scheduled task. If we get a resource here, the only images from invalidated_rect need to be decoded. And you'll need to map this rect to layer space similar to the content_rect (https://cs.chromium.org/chromium/src/cc/tiles/picture_layer_tiling.cc?dr&l=996).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c00fd9cee0bb99a69885bb83228e8cbdb63118a1

commit c00fd9cee0bb99a69885bb83228e8cbdb63118a1
Author: sohan <sohan.jyoti@huawei.com>
Date: Sat Oct 28 04:14:50 2017

cc: Decode images in invalidated rect.

For partial raster, only decode image in the invalidated rect
instead of the tiles content rect.

BUG= 767301 

Change-Id: I655b96f26913be2a52a90dde21a7701835e8fedd
Reviewed-on: https://chromium-review.googlesource.com/704927
Reviewed-by: Eric Karl <ericrk@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Commit-Queue: Sohan Jyoti Ghosh <sohan.jyoti@huawei.com>
Cr-Commit-Position: refs/heads/master@{#512383}
[modify] https://crrev.com/c00fd9cee0bb99a69885bb83228e8cbdb63118a1/cc/tiles/tile_manager.cc
[modify] https://crrev.com/c00fd9cee0bb99a69885bb83228e8cbdb63118a1/cc/tiles/tile_manager.h
[modify] https://crrev.com/c00fd9cee0bb99a69885bb83228e8cbdb63118a1/cc/tiles/tile_manager_unittest.cc