[fuchsia] Allow GPU watchdog to be disable via product config.

Debugging issues in the GPU process with zxdb is easier without the
watchdog running.

Change-Id: I9ff0e7fa8f00360ddf2b88475c17051207e6e5a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992539
Auto-Submit: Wez <wez@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729814}

fuchsia/engine/context_provider_impl.cc

@@ -150,6 +150,7 @@ bool MaybeAddCommandLineArgsFromConfig(const base::Value& config,
     return true;
 
   static const base::StringPiece kAllowedArgs[] = {
+      switches::kDisableGpuWatchdog,
       switches::kEnableFeatures,
       switches::kEnableFuchsiaAudioConsumer,
       switches::kEnableLowEndDeviceMode,