Roll shaderc from ff8fe2df3529 to 34c9aac3f30d (4 revisions)

https://chromium.googlesource.com/external/github.com/google/shaderc/+log/ff8fe2df3529..34c9aac3f30d

2020-07-23 dneto@google.com Start shaderc v2020.3 development
2020-07-23 dneto@google.com Finalize shaderc v2020.2
2020-07-23 dneto@google.com Update CHANGES
2020-07-23 dneto@google.com Roll third_party/glslang/ 9eef54b25..3ee5f2f1d (11 commits)

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/shaderc-chromium-autoroll
Please CC radial-bots+chrome-roll@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/master/autoroll/README.md

Tbr: radial-bots+chrome-roll@google.com
Change-Id: I4390ce79f9a9f989085929968a5502282eb37d87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2319114
Reviewed-by: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Commit-Queue: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#791483}