[Fuchsia] Add fuchsia.sysmem.Allocator to the runners' sandbox policy

fuchsia.sysmem.Allocator interface will be used to allocate buffers for
video frames.

Bug: Fuchsia: WEB-41
Change-Id: I08f2d23b31c8a2faa9422b1ecdc2ec45f4bfedfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609720
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659348}

fuchsia/runners/cast/sandbox_policy

@@ -9,6 +9,7 @@
       "fuchsia.net.SocketProvider",
       "fuchsia.netstack.Netstack",
       "fuchsia.process.Launcher",
+      "fuchsia.sysmem.Allocator",
       "fuchsia.ui.input.ImeService",
       "fuchsia.ui.input.ImeVisibilityService",
       "fuchsia.ui.scenic.Scenic",

fuchsia/runners/web/sandbox_policy

@@ -10,6 +10,7 @@
       "fuchsia.net.SocketProvider",
       "fuchsia.netstack.Netstack",
       "fuchsia.process.Launcher",
+      "fuchsia.sysmem.Allocator",
       "fuchsia.ui.input.ImeService",
       "fuchsia.ui.input.ImeVisibilityService",
       "fuchsia.ui.scenic.Scenic",