Add tests to verify the ARC app icon loading with large number apps.

This CL adds 2 test cases to simulate the ARC app icon loading when the
system start with 500 ARC apps.

1. The test case LoadLargeNumberIcons is used to verify adding 500 ARC
apps with good icons, then the system restarts, re-loading 500 ARC
app icons.

2. The test case LoadLargeNumberIconsWithBadIcon is used to verify
adding 500 ARC apps with good icons, and 10 ARC apps with bad icons,
then the system restarts, re-loading 510 ARC app icons.

Start 20 try jobs below, they both run pass.

BUG=1083331

Change-Id: I4d2f46757577e1db79d4aeb3d2aa497820dd0cf4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2389800


Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Long Cheng <lgcheng@google.com>
Cr-Commit-Position: refs/heads/master@{#805634}