Share logic for generating compile DB with run_tool.py

docs/clang_tool_refactoring.md

 # Clang Tool Refactoring
 
 [TOC]
 
 ## Introduction
 
 Clang tools can help with global refactorings of Chromium code. Clang tools can
 take advantage of clang's AST to perform refactorings that would be impossible
 with a traditional find-and-replace regexp:
 
@@ skipping 107 matching lines @@
 ninja -C out/Debug  # For non-Windows
 ninja -d keeprsp -C out/Debug  # For Windows
 
 # experimental alternative:
 $gen_targets = $(ninja -C out/gn -t targets all \
                  | grep '^gen/[^: ]*\.[ch][pc]*:' \
                  | cut -f 1 -d :`)
 ninja -C out/Debug $gen_targets
 ```
 
-On Windows, generate the compile DB first, and after making any source changes.
-Then omit the `--generate-compdb` in later steps.
-
-```shell
-tools/clang/scripts/generate_win_compdb.py out/Debug
-```
-
 Then run the actual clang tool to generate a list of edits:
 
 ```shell
 tools/clang/scripts/run_tool.py <toolname> \
   --generate-compdb
   out/Debug <path 1> <path 2> ... >/tmp/list-of-edits.debug
 ```
 
 `--generate-compdb` can be omitted if the compile DB was already generated and
 the list of build flags and source files has not changed since generation.
@@ skipping 65 matching lines @@
 
 ```shell
 tools/clang/scripts/test_tool.py <tool name>
 ```
 
 The name of the tool binary and the subdirectory for the tool in
 `//tools/clang` must match. The test runner finds all files that match the
 pattern `//tools/clang/<tool name>/tests/*-original.cc`, runs the tool across
 those files, and compared it to the `*-expected.cc` version. If there is a
 mismatch, the result is saved in `*-actual.cc`.