Roll src/third_party/shaderc/src a32acc1be500..9ad78aa7dba2 (3 commits)

https://chromium.googlesource.com/external/github.com/google/shaderc/+log/a32acc1be500..9ad78aa7dba2

git log a32acc1be500..9ad78aa7dba2 --date=short --first-parent --format='%ad %ae %s'
2020-01-22 rharrison@google.com Start v2020.0 (#970)
2020-01-22 rharrison@google.com Fix the version string (#971)
2020-01-22 rharrison@google.com Finalize v2019.1 (#969)

Created with:
  gclient setdep -r src/third_party/shaderc/src@9ad78aa7dba2

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/+/master/autoroll/README.md

Bug: None
Tbr: radial-bots+chrome-roll@google.com
Change-Id: Ia361fdd75bca63a2289eb72364aaba5ac8c446af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015447Reviewed-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@{#734230}

DEPS

@@ -308,7 +308,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling feed
   # and whatever else without interference from each other.
-  'shaderc_revision': 'a32acc1be500bdb204654169026721fc728e086d',
+  'shaderc_revision': '9ad78aa7dba268ef8b808d23b135ea636fd893b1',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling feed
   # and whatever else without interference from each other.