[DevTools] automapping should be less eager
[DevTools] automapping should be less eager

We need additional check otherwise auto mapping may request content for
dead UISourceCode.

R=73889f071ff743ad62763f13ff61c52ca3ee19d0@chromium.org

Bug: none
Change-Id: I098de1a6a157df3501cf969359168f6b92eeb2a0
Reviewed-on: https://chromium-review.googlesource.com/1166327
Reviewed-by: Andrey Lushnikov <73889f071ff743ad62763f13ff61c52ca3ee19d0@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <562acbea96970f508ad9f5cc9fc7c62d82dd3ef8@chromium.org>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#581469}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: aaebf9da51b8fbcdccdfebf45ae8acfe266e6268
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