Clobber the cros-chrome-sdk cache everytime when fetching artifacts.

Adds 2-3 minutes to every cros-flavored "gclient runhooks", which
is unfortunate.

Should be unnecessary when/if crbug.com/834134 is fixed.

Bug: 832374
Change-Id: I2696fc736350ecbe2b641112d2140db66118a00e
Reviewed-on: https://chromium-review.googlesource.com/1016137Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551850}

DEPS

@@ -2011,6 +2011,9 @@ hooks = [
       '--download-vm',
       '--board={cros_board}',
       '--cache-dir=src/build/cros_cache/',
+      # TODO(crbug.com/834134): Remove the cache clobber when the sdk is smart
+      # enough to eject old toolchains from the cache.
+      '--clear-sdk-cache',
       '--log-level=error',
       'exit',
     ],