tools/polymer/polymer.py: Fix invalid escape sequence SyntaxWarning

[155/51065] ACTION //chrome/browser/resources/browser_switch:app_module(//build/toolchain/win:win_clang_x64)
../../tools/polymer/polymer.py:307: SyntaxWarning: invalid escape sequence \(
  CR_DEFINE_START_REGEX = 'cr.define\('

Bug: 958138
Change-Id: I07a7b6fe2e4fe2e983b95e38c20c50986abd28e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1816410
Auto-Submit: Raul Tambre <raul@tambre.ee>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698979}

tools/polymer/polymer.py

@@ -304,7 +304,7 @@ def _process_dom_module(js_file, html_file):
   IIFE_CLOSING = '})();'
 
   # Remove this line.
-  CR_DEFINE_START_REGEX = 'cr.define\('
+  CR_DEFINE_START_REGEX = r'cr.define\('
   # Ignore all lines after this comment, including the line it appears on.
   CR_DEFINE_END_REGEX = r'\s*// #cr_define_end'