Set optional LastSwiftUpdateCheck to Xcode project attributes.
Set optional LastSwiftUpdateCheck to Xcode project attributes.

Adding this to also supress project upgrade warnings for Swift-based gyp generated project files.

R=mark@chromium.org

Review URL: https://codereview.chromium.org/1430573009 .

Patch from Alastair Tse <altse@chromium.org>.

adblockplus/gyp,turbulenz/gyp,chromium/gyp,pandaxcl/gyp,ttyangf/gyp,turbulenz/gyp,msc-/gyp,Phuehvk/gyp,Phuehvk/gyp,AWhetter/gyp,msc-/gyp,pandaxcl/gyp,ttyangf/gyp,msc-/gyp,turbulenz/gyp,AWhetter/gyp,chromium/gyp,AWhetter/gyp,pandaxcl/gyp,chromium/gyp,bnoordhuis/gyp,ttyangf/gyp,ttyangf/gyp,pandaxcl/gyp,turbulenz/gyp,Phuehvk/gyp,AWhetter/gyp,bnoordhuis/gyp,msc-/gyp,bnoordhuis/gyp,turbulenz/gyp,adblockplus/gyp,ttyangf/gyp,adblockplus/gyp,Phuehvk/gyp,adblockplus/gyp,AWhetter/gyp,chromium/gyp,Phuehvk/gyp,bnoordhuis/gyp,msc-/gyp,bnoordhuis/gyp,pandaxcl/gyp,adblockplus/gyp,chromium/gyp