Roll SwiftShader 71e256c..847fc0b

https://swiftshader.googlesource.com/SwiftShader.git/+log/71e256c..847fc0b

BUG=chromium:881334

TBR=kbr@chromium.org

TEST=bots

CQ_INCLUDE_TRYBOTS=luci.chromium.try:win_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:android_optional_gpu_tests_rel

Change-Id: I04a81f1daaa962a3a2510d76784dcb3e0f704e17
Reviewed-on: https://chromium-review.googlesource.com/c/1285480
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Alexis Hétu <sugoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600353}

DEPS

@@ -133,7 +133,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling SwiftShader
   # and whatever else without interference from each other.
-  'swiftshader_revision': '71e256cae02ac7e15c61c1925ac4a43c102e23b1',
+  'swiftshader_revision': '847fc0b278f8454f8e04c89163bf840aa903df99',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling PDFium
   # and whatever else without interference from each other.