Migrate coverage_report.py to use upstream Chromium scripts

This adds tools/code_coverage from Chromium to DEPS and converts our
existing coverage_report.py to use it instead of gcov & lcov. This
generates a different format of HTML report, but the content appears
to be the same. Some of the coverage numbers changed a bit, due to
differences in how llvm-cov and gcov calculate executable lines, but
drilling down into the reports I think llvm-cov is more accurate
overall and there are no major discrepancies.

Large portions of the existing script are left as is and just the
report generation has been changed. I plan in follow up CLs to remove
the duplication of functionality in the PDFium scripts and modularlize
the upstream code better.

BUG=pdfium:1069

Change-Id: I009bfb8aac8f1a878e01ff70923e19bbb4774a9c
Reviewed-on: https://pdfium-review.googlesource.com/32894
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/DEPS b/DEPS
index a7f5e7c..ed4d7b3 100644
--- a/DEPS
+++ b/DEPS
@@ -19,6 +19,7 @@
   'buildtools_revision': '893eb86b02b2571894e328f05551112b96df1cce',
   'catapult_revision': '8fbdf9f10b76595b6299e577709f2608ae29e19a',
   'clang_revision': 'c893c7eec4706f8c7fc244ee254b1dadd8f8d158',
+  'code_coverage_revision': '14b8501b7d9f41826d632254a1df8bc664edeec7',
   'cygwin_revision': 'c89e446b273697fadf3a10ff1007a97c0b7de6df',
   'depot_tools_revision': '869a2857a4f3acc08fdec6182ecec5b73593be3b',
   'freetype_revision': 'd45d4b97e6a03cfe21a2952138cf6e970fc358e5',
@@ -102,6 +103,10 @@
   "tools/clang":
     Var('chromium_git') + "/chromium/src/tools/clang@" +  Var('clang_revision'),
 
+  "tools/code_coverage":
+    Var('chromium_git') + "/chromium/src/tools/code_coverage.git@" +
+        Var('code_coverage_revision'),
+
   # TODO(GYP): Remove this when no tools rely on GYP anymore.
   "tools/gyp":
     Var('chromium_git') + '/external/gyp.git@' +