[Fuchsia] Update layout_test flags for content_shell

Added the following flags for content_shell when running layout tests:
--use-vulkan --enable-gpu-rasterization --force-device-scale-factor=1
This ensures that the web tests flag on Fuchsia matches the flags used
in WebEngine.

Bug: 1067477
Change-Id: I39fe18d4a70cf76f6f422173c65403ede70379fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446541
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814258}

third_party/blink/tools/blinkpy/web_tests/port/fuchsia.py

@@ -331,7 +331,9 @@ class ChromiumFuchsiaDriver(driver.Driver):
         elif self._port._target_device == 'aemu':
             cmd.extend([
                 '--ozone-platform=scenic', '--enable-oop-rasterization',
-                '--use-gl=stub', '--enable-features=UseSkiaRenderer,Vulkan'
+                '--use-vulkan', '--enable-gpu-rasterization',
+                '--force-device-scale-factor=1', '--use-gl=stub',
+                '--enable-features=UseSkiaRenderer,Vulkan'
             ])
         return cmd