Build pdfium_diff for coverage tests.

Otherwise, the coverage tool runs pdfium_diff, and gets confused because
it shares object files with the other binaries, but was built at a
different time.

Change-Id: Iddbd02f12e75f1ae62b0668cb638ece30c7aaf63
Reviewed-on: https://pdfium-review.googlesource.com/c/46070
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>

testing/tools/coverage/coverage_report.py

diff --git a/testing/tools/coverage/coverage_report.py b/testing/tools/coverage/coverage_report.py
index 76a6a61..d6b6f80 100755
--- a/testing/tools/coverage/coverage_report.py
+++ b/testing/tools/coverage/coverage_report.py
@@ -142,6 +142,7 @@
       test_spec = COVERAGE_TESTS[name]
       if test_spec.use_test_runner:
         binary_path = os.path.join(testing_tools_directory, test_spec.binary)
+        build_targets.add('pdfium_diff')
         build_targets.add('pdfium_test')
       else:
         binary_path = os.path.join(self.build_directory, test_spec.binary)