platform/manifest (marshmallow ) - set clone-depth=1 for prebuilts/chromium-webview

[Arnav Gupta]

For building Android, we only need the tip of the head of chromium-webview, not a history of all earlier compiled revisions.

I suggest in platform/manifest, the chromium-webview project's clone-depth be set to 1.

I tried pushing this for review, but it turns out, people without outside Googleplex can't push changes for review to the manifest repo.