Fix a Chrome crash where TopSitesImpl::AddForcedURL would access the top sites cache_ before it finished loading.
Fix a Chrome crash where TopSitesImpl::AddForcedURL would access the top sites cache_ before it finished loading.

Verified manually by forcing the cache_ not to load and was able to repro the crash and to check that the fix works.

BUG=359987
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/1231633002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#338060}

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