Bug 1111957 - Invalidate mDPI and mDefaultScale in TabChild when switching displays. r=fabrice.

Bug 1107259 made TabChild cache the display DPI and default scale more aggressively, and
we need to invalidate those caches when switching displays, otherwise we can get weird
scaling behaviours when switching between displays with different DPIs and scales.

--HG--
extra : rebase_source : cccbdae40113ae1a1b76d020ddc93e7b393fce61
extra : amend_source : d2664b4ea30342476e67da2a16315be4c1920abf