String change on CrOS notification for presenting

Change the string on CrOS notification for presenting to "You're sharing
your screen" from "You are sharing your screen", which is from UI review
for changing source feature,
https://docs.google.com/presentation/d/1AgftDesmQ4BbsDDdC8Dahox472OMSkadwE51oYmlfeI/edit#slide=id.g4f9c055e47_0_150.

Bug: 941028
Change-Id: I7f0f7b12fbd741ba14707208d74b99099be33a0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529750
Commit-Queue: Weiyong Yao <braveyao@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643141}
1 file changed