[Extensions][TaskMigration] Remove usage of BrowserThread in ActivityLog

Remove the checks if certain BrowserThreads exist in ActivityLog. These
used to be necessary since tasks ran on certain threads that might not
exist in unittests, but are no longer needed.

Also remove some debug logs that previously crept in.

Bug: 689520
Change-Id: I2065ba6c01a84b5f3c05dc82028fdabc02738558
Reviewed-on: https://chromium-review.googlesource.com/621406
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495620}
3 files changed