Fix crash in is_cfi=true builds with unbundled ICU

Ensure ICU symbols have public visibility and are thus excluded from CFI
checks and whole-program optimization. The former caused a startup crash
and the latter has the potential to break virtual calls in weird ways.

BUG=822820

Change-Id: Ia809eefcb9e93b3c612f2381d394db83bbc67120
Reviewed-on: https://chromium-review.googlesource.com/978008Reviewed-by: Peter Collingbourne <pcc@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545638}

build/linux/unbundle/icu.gn

@@ -17,6 +17,24 @@ config("icu_config") {
     "USING_SYSTEM_ICU=1",
     "ICU_UTIL_DATA_IMPL=ICU_UTIL_DATA_STATIC",
     "UCHAR_TYPE=uint16_t",
+
+    # U_EXPORT (defined in unicode/platform.h) is used to set public visibility
+    # on classes through the U_COMMON_API and U_I18N_API macros (among others).
+    # When linking against the system ICU library, we want its symbols to have
+    # public LTO visibility. This disables CFI checks for the ICU classes and
+    # allows whole-program optimization to be applied to the rest of Chromium.
+    #
+    # Both U_COMMON_API and U_I18N_API macros would be defined to U_EXPORT only
+    # when U_COMBINED_IMPLEMENTATION is defined (see unicode/utypes.h). Because
+    # we override the default system UCHAR_TYPE (char16_t), it is not possible
+    # to use U_COMBINED_IMPLEMENTATION at this moment, meaning the U_COMMON_API
+    # and U_I18N_API macros are set to U_IMPORT which is an empty definition.
+    #
+    # Until building with UCHAR_TYPE=char16_t is supported, one way to apply
+    # public visibility (and thus public LTO visibility) to all ICU classes is
+    # to define U_IMPORT to have the same value as U_EXPORT. For more details,
+    # please see: https://crbug.com/822820
+    "U_IMPORT=U_EXPORT",
   ]
 }