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This fixes 2 bugs which could result in using invalid textures from the
image cache, if an image is originally uploaded at original size and
mips are generated with a subsequent copy later.

1) When we generate mips the original texture is deleted, while if the
DrawImage is ref-ed and there is an external ref on the SkImage backed
by the original texture it could potentially be used after its deleted.

2) If skia fails to mip the texture for any reason, the "mipped" image
returned is backed by the original texture which we would subsequently
delete.

R=ccameron@chromium.org, ericrk@chromium.org
TBR=piman@chromium.org

Bug: 870317
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Iec925f35c7880db89f9b68a9cee5e0b76b8ce4d9
Reviewed-on: https://chromium-review.googlesource.com/1171960Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582484}