[code coverage] Normalize source file path in compile wrapper.

Paths in `--files-to-instrument` are expected to be normalized OS-native
paths, so the input source file has to be converted to match.

This is most relevant on Windows, where GN creates build commands that
use `/` for filesystem paths, but the canonical form should use `\`.

Bug: 990480
Change-Id: If7b5da729f0a98ec7945092b0f228cebd50be3a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1973360
Commit-Queue: Sajjad Mirza <sajjadm@chromium.org>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Roberto Carrillo <robertocn@chromium.org>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726120}

build/toolchain/clang_code_coverage_wrapper.py

@@ -152,7 +152,11 @@ def main():
   if source_flag_index + 1 >= len(compile_command):
     raise Exception('Source file to be compiled is missing from the command.')
 
-  compile_source_file = compile_command[source_flag_index + 1]
+  # On Windows, filesystem paths should use '\', but GN creates build commands
+  # that use '/'. We invoke os.path.normpath to ensure that the path uses the
+  # correct separator for the current platform (i.e. '\' on Windows and '/'
+  # otherwise).
+  compile_source_file = os.path.normpath(compile_command[source_flag_index + 1])
   exclusion_list = _COVERAGE_EXCLUSION_LIST_MAP.get(
       target_os, _DEFAULT_COVERAGE_EXCLUSION_LIST)