Disable framebuffer discarding on Mali-4xx and clean up.

It seems all of the known bugs with Mali-400 are related to framebuffer
discarding.  Blacklist framebuffer discarding universally on Mali-400
and reenable ganesh and canvas where it was still blacklisted on Mali-400.

There was a preexisting workaround applied to all ARM < 4.4 which was
originally written in the context of Mali-T604; this explains why the
bugs only reproed on some Mali-400 devices.  Add a new one for all 4xx.

Also, a recently added Nexus 6 workaround did essentially the same
thing, but at the level of making the discard do nothing instead of not
advertising the extension.  In OpenGL ES 3, this is a core method so
failing to advertise won't be sufficient, so merge the two workarounds
and remove the "_ext_" from the name.

BUG=449488,451230

Review URL: https://codereview.chromium.org/1068573004

Cr-Commit-Position: refs/heads/master@{#324152}
(cherry picked from commit 8ef99b8a51842a038c64cd2b1ab1896228651ec2)

Review URL: https://codereview.chromium.org/1070493002

Cr-Commit-Position: refs/branch-heads/2357@{#12}
Cr-Branched-From: 59d4494849b405682265ed5d3f5164573b9a939b-refs/heads/master@{#323860}

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