Disable focused node changed notification for a11y in --site-per-process.
Disable focused node changed notification for a11y in --site-per-process.

Since the removal of swapped out RenderFrame in --site-per-process, it is
no longer correct to assume that RenderView::GetMainRenderFrame() will
return a non-null value.

There is ongoing work to refactor accessibility to fit better with
out-of-process iframes, which will help fix this issue properly. Until
this work is complete, work around the crash by no longer notifying the
accessibility subsystem of focused node changes, as the code assumes the
existence of main frame in a subframe process.

BUG=497839, 498851
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation

Review URL: https://codereview.chromium.org/1266313003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#342244}

Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,Just-D/chromium-1,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,Just-D/chromium-1,Chilledheart/chromium,Chilledheart/chromium,Just-D/chromium-1,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,Chilledheart/chromium,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk