Increase theme version to fix M57->M58 upgrade not migrating themes.

chrome/browser/themes/browser_theme_pack.cc

Index: chrome/browser/themes/browser_theme_pack.cc
diff --git a/chrome/browser/themes/browser_theme_pack.cc b/chrome/browser/themes/browser_theme_pack.cc
index b256778dd8665f3d56cd213d98bd278ce762cef9..9205d3a9bfe0e5747dc1fe4c43f347b1f70423e9 100644
--- a/chrome/browser/themes/browser_theme_pack.cc
+++ b/chrome/browser/themes/browser_theme_pack.cc
@@ -48,7 +48,7 @@ namespace {
 // theme packs that aren't int-equal to this. Increment this number if you
 // change default theme assets or if you need themes to recreate their generated
 // images (which are cached).
-const int kThemePackVersion = 47;
+const int kThemePackVersion = 48;
 
 // IDs that are in the DataPack won't clash with the positive integer
 // uint16_t. kHeaderID should always have the maximum value because we want the