[fuchsia] Add --enable-hardware-overlays=underlay to WebEngine

Flatland currently does not support Fullscren or SingleOnTop overlay
strategies.

Bug: 1449048, b/283290297
Change-Id: I4dae9a310d426553cdde9734184f945945c2c850
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4568129
Commit-Queue: David Song <wintermelons@google.com>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149846}

fuchsia_web/webengine/browser/web_engine_config.cc

@@ -160,5 +160,9 @@ bool UpdateCommandLineFromConfigFile(const base::Value::Dict& config,
     }
   }
 
+  // TODO(crbug.com/1449048): Remove this switch once fixed.
+  command_line->AppendSwitchASCII(switches::kEnableHardwareOverlays,
+                                  "underlay");
+
   return true;
 }