fix: backport ICU changes for Japanese Era placeholder name (#17518)

This change backports changes from the upstream ICU 62.1 maintenance
branch into the Chromium copy of ICU, in order to enable the
"placeholder era" for the upcoming new Japanese Era.

Upstream change: unicode-org/icu#514
Upstream ticket: https://unicode-org.atlassian.net/browse/ICU-20482

With this change you can set/define an Environment Variable
`ICU_ENABLE_TENTATIVE_ERA` to `true which will enable the placeholder
era name.

Note: This change requires re-generating the checked-in Chromium ICU
data files (icudtl.dat) in order for them to have the updated data
for the placeholder era name.

Also included in this change are some minor changes to the scripts
used by Chromium for building the data files so that they actually
work on Ubuntu.

patches/common/config.json

@@ -5,6 +5,8 @@
 
   "src/electron/patches/common/ffmpeg": "src/third_party/ffmpeg",
 
+  "src/electron/patches/common/icu": "src/third_party/icu",
+
   "src/electron/patches/common/skia": "src/third_party/skia",
 
   "src/electron/patches/common/webrtc":  "src/third_party/webrtc",

patches/common/icu/.patches

@@ -0,0 +1 @@
+icu_changes_for_japanese_era_placeholder_name_support.patch