Merge remote-tracking branch 'aosp/upstream' into master

b9fb0ad5 Merge pull request #119 from colincross/extend
bf2adbfe Relax type requirements when extending properties
7fda24e5 Merge pull request #118 from mortonfox/patch-1
ebb35417 Update link to Ninja
e412f09f Merge pull request #117 from colincross/variant
badc8817 Elide empty variations

Change-Id: If24af2b0fb0b5772597ded86a26dfd98f85d8430