Fix regression in coverage_report.py

Restore the original way that coverage_report.py was calling
subprocess.call(). Before https://pdfium-review.googlesource.com/117451,
the arguments were:

['ninja', ..., 'target1', ..., 'targetN'].

Now, they unintentionally became:

['autoninja', ..., ['target1', ..., 'targetN']].

Remove the extra nested list to avoid Python exceptions from the
subprocess module.

Change-Id: Ieab165b3cc206bff26ee105d5a7b9832ec46a721
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/117670
Reviewed-by: Thomas Sepez <tsepez@google.com>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/testing/tools/coverage/coverage_report.py b/testing/tools/coverage/coverage_report.py
index 2f3ba99..0b5641c 100755
--- a/testing/tools/coverage/coverage_report.py
+++ b/testing/tools/coverage/coverage_report.py
@@ -177,7 +177,8 @@
   def build_binaries(self):
     """Build all the binaries that are going to be needed for coverage
     generation."""
-    call_args = ['autoninja', '-C', self.build_directory, self.build_targets]
+    call_args = ['autoninja', '-C', self.build_directory]
+    call_args.extend(self.build_targets)
     return self.call(call_args, dry_run=self.dry_run) == 0
 
   def generate_coverage(self, name, spec):