Add a DEPS hook to download llvm-cov and llvm-profdata based on .gclient custom_vars.

Bug: b/236797073
Change-Id: I8f72240a2c4ca0b47d431598fb70e3319e9675b8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/292420
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39263}

DEPS

@@ -12,6 +12,11 @@ vars = {
   'checkout_instrumented_libraries': 'checkout_linux and checkout_configuration == "default"',
   'chromium_revision': 'e182675fbb4214865ca583000ac647bef0b8d9d6',
 
+  # Fetch the prebuilt binaries for llvm-cov and llvm-profdata. Needed to
+  # process the raw profiles produced by instrumented targets (built with
+  # the gn arg 'use_clang_coverage').
+  'checkout_clang_coverage_tools': False,
+
   # Keep the Chromium default of generating location tags.
   'generate_location_tags': True,
 
@@ -2447,6 +2452,14 @@ hooks = [
     'pattern': '.',
     'action': ['python3', 'src/tools/clang/scripts/update.py'],
   },
+  {
+    # This is supposed to support the same set of platforms as 'clang' above.
+    'name': 'clang_coverage',
+    'pattern': '.',
+    'condition': 'checkout_clang_coverage_tools',
+    'action': ['python3', 'src/tools/clang/scripts/update.py',
+               '--package=coverage_tools'],
+  },
   {
     # Update LASTCHANGE.
     'name': 'lastchange',