Add DEPS condition vars for GCS dep autoroller

An autoroller will be set up soon for pdfium to keep chromium-side GCS
deps (like third_party/llvm-build/Release+Asserts and
third_partyrust-toolchain) in sync with pdfium's GCS deps. Chromium's
Release+Asserts dep uses additional conditional vars, which pdfium
doesn't use, but is needed to be defined because the autoroller will
naively copy all the dep's objects and conditions over.

Bug: 324418194
Change-Id: I044ff3706801487b29c73be8a6040b0b2d5695d3
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/124531
Commit-Queue: Stephanie Kim <kimstephanie@google.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>

DEPS

diff --git a/DEPS b/DEPS
index e1b2917..154ec82 100644
--- a/DEPS
+++ b/DEPS
@@ -26,6 +26,12 @@
   # support libraries.
   'checkout_android_native_support': 'checkout_android',
 
+  'checkout_clang_coverage_tools': 'False',
+
+  'checkout_clang_tidy': 'False',
+
+  'checkout_clangd': 'False',
+
   'checkout_instrumented_libraries': 'checkout_linux and checkout_configuration != "small" and checkout_configuration != "minimal"',
 
   # Fetch the rust toolchain.