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}
(cherry picked from commit 71b32fcc875526596305a98c1a1194e690dc8719)

TBR=sergeyu@chromium.org

Committed: https://chromium.googlesource.com/chromium/src/+/6debe587e7d6c302b98f287d1b090a50a97025e3

[Sergey Ulanov, 2014-09-02 18:38:12]

Committed patchset #1 (id:1) manually as 6debe58 (presubmit successful).