Add unit tests for ExtensionInstallEventLogManager

ExtensionInstallEventLogManager class ties together collection, storage
and upload of extension logs. This is used for event based reporting
for policy based extensions. Add unit tests for this class. This CL
add unit tests for ExtensionInstallEventLogManager.

Bug: 1101709
Change-Id: Iff4ef1a5af9f979336fba5073426cb6313912f8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2277817Reviewed-by: Sergey Poromov <poromov@chromium.org>
Commit-Queue: Swapnil Gupta <swapnilgupta@google.com>
Cr-Commit-Position: refs/heads/master@{#785311}

chrome/browser/chromeos/BUILD.gn

@@ -3268,6 +3268,7 @@ source_set("unit_tests") {
     "policy/dm_token_storage_unittest.cc",
     "policy/extension_cache_unittest.cc",
     "policy/extension_install_event_log_collector_unittest.cc",
+    "policy/extension_install_event_log_manager_unittest.cc",
     "policy/extension_install_event_log_unittest.cc",
     "policy/extension_install_event_log_uploader_unittest.cc",
     "policy/extension_install_event_logger_unittest.cc",