Fixes compile error

Without this I get compile errors in motion_event_web.cc because M_PI
is undeclared.

BUG=409940
TEST=none
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/533863002

Cr-Commit-Position: refs/heads/master@{#292991}

build/precompile.h

@@ -20,6 +20,8 @@
 
 #define BUILD_PRECOMPILE_H_
 
+#define _USE_MATH_DEFINES
+
 // The Windows header needs to come before almost all the other
 // Windows-specific headers.
 #include <Windows.h>