[code coverage] Make source path starts with "//"

The source path should start with "//":
https://cs.chromium.org/chromium/infra/appengine/findit/model/proto/code_coverage.proto?q=code_coverage.pro&sq=package:chromium&g=0&l=91-92

Bug: 981020
Change-Id: I5e0119f6575332ae507652005b2979721ff23b6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1696446
Commit-Queue: Yun Liu <yliuyliu@google.com>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676191}

build/android/generate_jacoco_report.py

@@ -191,7 +191,8 @@ def _GenerateJsonCoverageMetadata(out_file_path, jacoco_xml_path, source_dirs):
       print('Processing file %s' % path)
 
       file_coverage = {}
-      file_coverage['path'] = path
+      # Source path needs to start with '//', https://bit.ly/2XC80ZL
+      file_coverage['path'] = '//' + path
       file_coverage['lines'] = []
       file_coverage['branches'] = []