Add internal preinstalled_web_apps directory to .gitignore

This CL adds chrome/browser/resources/preinstalled_web_apps/internal to
.gitignore in preparation for adding it to src-internal:
https://chrome-internal-review.googlesource.com/c/chrome/src-internal/+/3402544

No-Try: true
Bug: 1147692
Change-Id: I2b42c5c9b1b6c5da267366dbf6354be7fafc6859
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537274
Reviewed-by: Glen Robertson <glenrob@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827184}

.gitignore

@@ -110,6 +110,7 @@ vs-chromium-project.txt
 /chrome/browser/resources/chromeos/quickoffice
 /chrome/browser/resources/pdf/html_office
 /chrome/browser/resources/media_router/extension/src/
+/chrome/browser/resources/preinstalled_web_apps/internal/
 /chrome/browser/resources/media_router_internal/
 /chrome/browser/resources/settings_internal/
 /chrome/browser/resources/software_rendering_list