[fuchsia] Add fuchsia.memorypressure.Provider to test sandboxes.

Add fuchsia.memorypressure.Provider to the test sandbox, so that test
suites don't report errors connecting to it.

Bug: 1020698
Change-Id: I15f7a0b3faaddf973c22fd72f215f533c00be695
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2072217
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745686}

build/config/fuchsia/tests-with-exec.cmx

@@ -22,6 +22,7 @@
       "fuchsia.media.SessionAudioConsumerFactory",
       "fuchsia.media.drm.Widevine",
       "fuchsia.mediacodec.CodecFactory",
+      "fuchsia.memorypressure.Provider",
       "fuchsia.net.NameLookup",
       "fuchsia.netstack.Netstack",
       "fuchsia.posix.socket.Provider",

build/config/fuchsia/tests.cmx

@@ -22,6 +22,7 @@
       "fuchsia.media.SessionAudioConsumerFactory",
       "fuchsia.media.drm.Widevine",
       "fuchsia.mediacodec.CodecFactory",
+      "fuchsia.memorypressure.Provider",
       "fuchsia.net.NameLookup",
       "fuchsia.netstack.Netstack",
       "fuchsia.posix.socket.Provider",