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Makes waitForMainTabCount: print the actual tab count when it fails.

Bug: none
Change-Id: I63e3b26b33c0cede5c8bec72b4f35b738314fa6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1942341
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727458}