[desktopCapture] set button_OK as primary

In the past we set button_cancel as the default button in meida picker.
According to the latest discussion, https://docs.google.com/presentation/d/1AgftDesmQ4BbsDDdC8Dahox472OMSkadwE51oYmlfeI/edit#slide=id.g5059ee7351_0_58,
reverse the primary/sencondary button now.

Bug: 941026
Change-Id: I82572814b5ae34be36ee6de721b126bafcb834c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529737
Reviewed-by: Christopher Thompson <[email protected]>
Commit-Queue: Weiyong Yao <[email protected]>
Cr-Commit-Position: refs/heads/master@{#642234}
2 files changed