Combine two jacoco reports.

Combines the report generated from the host (junit) code coverage
with the report generated from the test a ran on device. It goes to each
method and updates the counters from whichever coverage is higher.

It then updates the lines covered in the source file and finally,
updates the counters.

Please note if an instruction has branch A,B,C, and one report covers
only branch A and another only covers Branch B and C, it will choose the
report results that cover branch B and C, showing A as uncovered. This
is because if one report covered A and B, and another covered B, we
can't say "3 branches are covered."

Bug: 1107004
Change-Id: I8dfa9feb80df5c9ed6d468226c25155383962519
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2333065
Commit-Queue: benjamin joyce <bjoyce@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796081}

build/android/PRESUBMIT.py

@@ -73,6 +73,7 @@ def CommonChecks(input_api, output_api):
           output_api,
           unit_tests=[
               J('.', 'emma_coverage_stats_test.py'),
+              J('.', 'generate_jacoco_report_test.py'),
               J('.', 'list_class_verification_failures_test.py'),
               J('gyp', 'util', 'build_utils_test.py'),
               J('gyp', 'util', 'manifest_utils_test.py'),