Add missing dependency causing the build to flakily fail.

The failure could be easily reproduced as follows

rm -rf out/<out>/gen/chrome/browser/resources
rm -rf out/<out>/ui/webui/resources
ninja -C out/<out> chrome/browser/resources/extensions:build

and was caused by a missing dependency to cr_components:preprocess.

TBR=futhark@chromium.org

No-Tree-Checks: true
No-Try: true
Bug: 1143476
Change-Id: Ib06568eeb9782beace3093cb434c58c978708d4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2506643
Commit-Queue: dpapad <dpapad@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822069}

ui/webui/resources/BUILD.gn

@@ -41,6 +41,7 @@ group("preprocess") {
   deps = [
     ":preprocess_generated",
     ":preprocess_src",
+    "cr_components:preprocess",
     "cr_elements:preprocess",
   ]
 }