Don't send error messages for touches on iframes
Don't send error messages for touches on iframes

This is a normal case, so there's no reason for it to be treated as an
error. Sending a message causes an expensive JS->ObjC communication (in
fact, quite a few due to the way the touch looks nearby for elements),
which hurts gesture responsiveness.

BUG=532207

Review URL: https://codereview.chromium.org/1344953002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#349013}

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