Prepare for |is_posix| switch in the Fuchsia build

|is_posix| will be switched to false for Fuchsia, this is a preliminary change.

Bug: chromium:812974
Change-Id: I2ae8e5ef4270cd0acbc7c69bb46d1be9ed4a5c5e
Reviewed-on: https://chromium-review.googlesource.com/988253Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52407}

BUILD.gn

@@ -2698,7 +2698,7 @@ v8_component("v8_libbase") {
     defines = [ "BUILDING_V8_BASE_SHARED" ]
   }
 
-  if (is_posix) {
+  if (is_posix || is_fuchsia) {
     sources += [
       "src/base/platform/platform-posix.cc",
       "src/base/platform/platform-posix.h",
@@ -3143,7 +3143,7 @@ v8_executable("d8") {
     "//build/win:default_exe_manifest",
   ]
 
-  if (is_posix) {
+  if (is_posix || is_fuchsia) {
     sources += [ "src/d8-posix.cc" ]
   } else if (is_win) {
     sources += [ "src/d8-windows.cc" ]