[telemetry] Attempt to fix testActivateTab.

The test immediately checks for document visible. It might not be true because the page hasn't finished loading yet. So we should just wait for it.


BUG=326725
TEST=tools/telemetry/run_tests testActivateTab  # Though this isn't a 100% test, since the flakiness was rare and Windows-only.
TBR=tonyg

Review URL: https://codereview.chromium.org/110623002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241193 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241193
Don't set file_externally_removed_ if DeleteDownloadedFile failed.

Tested on linux by:
mkdir Downloads/x/
save a file to Downloads/x/x.txt in chrome
sudo chown root:root ~/Downloads/x
sudo chmod 755 ~/Downloads/x
removeFile() still doesn't return an error message, but at least it doesn't set exists=false.
If you chmod 700 ~/Downloads/x then chrome can't find the file so it assumes it was deleted. I'm not sure if windows would have a similar situation since its ACLs work a little differently.

Perhaps another CL will plumb the error to removeFile's callback.

BUG=321305

Review URL: https://codereview.chromium.org/110863006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241194 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241194