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As a second step to convert the content shell test harness, this CL
converts the below messages to Mojo by using WebTestClient interface.

 - WebTestHostMsg_SimulateWebNotificationClose
 - WebTestHostMsg_SimulateWebContentIndexDelete
 - WebTestHostMsg_InspectSecondaryWindow
 - WebTestHostMsg_TestFinishedInSecondaryRenderer

WebTestClientImpl class implements the methods of
WebTestClient interface.

Other WebTestHostMsg conversions will be prepared after this
initial CL.

Bug: 1039247
Change-Id: I3f021e7c17a5239c4eb1467d464a312411347b12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032697Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#738797}