Set mac_deployment_target default to 10.7
Set mac_deployment_target default to 10.7

This overrides the default (10.6) in Chromium's
build/common.gypi. It's needed since we want ARC and libc++.

TESTED=Ran webrtc/build/gyp_webrtc before this patch and then
grep -r macosx-version-min out/Debug/* | grep 10.6
which gave a lot of output.
Then with this patch applied, there were no output for 10.6 (only 10.7).

R=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1474623002 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10783}

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