Recompute webpage viewport dimensions when resizing html frames

This patch ensures the viewport dimensions of the composer html frame are recomputed every time the frame is resized, to ensure that the webbpage is always rendered according to the available space.

Funded by Sourcepole QGIS Enterprise.
This can't be called before the viewport size has been calculated, as it requires the viewport. Otherwise page break calculation will be thrown out. Instead, it should be called before QgsComposerMultiFrame::recalculateFrameSizes() in  QgsComposerHtml::recalculateFrameSizes.