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This change causes the desktop media picker not to use TabbedPane
unless there are multiple source types to switch between - otherwise
it presents an odd single-tab TabbedPane. Specifically, this change:

1) Renames the pane_ member of DesktopMediaPickerDialogView and makes
   it optional;
2) Refactors the constructor of that class to add the created views
   children of the TabbedPane only when there are >1, and otherwise as
   direct children of the DialogView itself;
3) Overrides GetClassName on that class to aid debugging/inspection;
4) Adds new strings (from discussion with UX) for the dialog title when
   showing only a single source type;
5) Has DesktopMediaPickerDialogView::GetWindowTitle() use those new
   strings when appropriate;
6) Makes DesktopMediaPickerViewsBrowserTest's InvokeUi test fill
   itself with some test data, as an aid to screenshots;
7) Adds a new InvokeUi test to that suite, named "tabs", which shows
   the UI with only a single source;
8) Adds a new test to that suite, SingleSourceTypeChangesTitle, which
   validates the changes from (4) & (5);
9) Overrides DesktopMediaTabList::GetHeightForWidth, which is required
   for it to lay out properly when contained in a BoxLayout directly
   rather than a TabbedPane;

To test this change, use the same steps as CL 1626010, or use InvokeUi.

Bug: 964332
Change-Id: I0ba7f47613cfbd76152e7ecbdad34b91230b603c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627919
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Auto-Submit: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663173}