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[Test] Add incognito toggle tests for GTS

This CL adds the following tests for incognito toggling in GTS.
  1. testIncognitoToggle_tabCount
  2. testIncognitoToggle_thumbnailFetchCount

testIncognitoToggle_thumbnailFetchCount is disabled for now, since the
current implemented behavior for thumbnail fetching is not ideal. Seems
like we fetch thumbnail for all tabs in the TabModel (normal+incognito),
when we switch TabModel or enter the tab switcher.

Change-Id: Ic6406ec14ec4a2c909eb025da38af988420eaf06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1856722
Commit-Queue: Mei Liang <meiliang@chromium.org>
Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707607}