Roll Fuchsia SDK to ff6b8e980b4e5b0c898341e8a467b9c751857e5d

Reverts the QEMU binary which caused a substantial perf regression on
arm64 emu.

TBR: wez@chromium.org
Bug: 780096, 707030
Change-Id: Ib48849dcfb261205a6211c9eec0a9e2b997970cd
Reviewed-on: https://chromium-review.googlesource.com/748682
Commit-Queue: Scott Graham <scottmg@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513088}

DEPS

@@ -1436,7 +1436,7 @@ hooks = [
     'action': [
       'python',
       'src/build/fuchsia/update_sdk.py',
-      'f6dffb2fee82a21900fc3a00261dc5844901ea9e',
+      'ff6b8e980b4e5b0c898341e8a467b9c751857e5d',
     ],
   },