Fix DesktopMediaPickerViews to handle background parent_web_contents

Fix DesktopMediaPickerViews to handle background parent_web_contents

After r283702 DesktopMediaPickerViews is getting parent_web_contents even
when it corresponds to a background extension page.
DesktopMediaPickerViews was always trying to create a modal dialog when
web_contents is specified, and that doesn't work for background pages.
Now it explicitly checks if the web_contents is a background page and
in that case shows the picker in a separate window.

Also updated example for desktop capture API to make it easier to test
this scenario.

BUG=402579
Review URL: https://codereview.chromium.org/501713002

Cr-Commit-Position: refs/heads/master@{#292055}

TBR=sergeyu@chromium.org

Committed: https://chromium.googlesource.com/chromium/src/+/fa2d62218b6c77a6bcd025e7055a609ddb8899bb

[Sergey Ulanov, 2014-09-03 02:01:10]

Committed patchset #1 (id:1) manually as fa2d622 (presubmit successful).