chart2: add DocumentBaseURL in ChartFrameLoader::load()

... as seen in JunitTest_chart2_unoapi

Change-Id: Ibe0cd210b8f2ddf730483705292e9027654b6fd4

chart2/source/controller/main/ChartFrameloader.cxx

@@ -165,6 +165,11 @@ sal_Bool SAL_CALL ChartFrameLoader::load( const uno::Sequence< beans::PropertyVa
                 }
                 else
                 {
+                    // use the URL as BaseURL, similar to what SfxBaseModel effectively does
+                    if (!aURL.isEmpty())
+                    {
+                        aMediaDescriptor[utl::MediaDescriptor::PROP_DOCUMENTBASEURL()] <<= aURL;
+                    }
                     aMediaDescriptor.addInputStream();
                     uno::Sequence< beans::PropertyValue > aCompleteMediaDescriptor;
                     aMediaDescriptor >> aCompleteMediaDescriptor;