Fixing bug in logic to ensure fastbuild=1
Fixing bug in logic to ensure fastbuild=1

When another gclient_env variable was set but not GYP_DEFINES, the whole gclient_env dict was overwritten instead of just appending fastbuild=1 to the GYP_DEFINES key.

BUG=none
TEST=local builds.

Review URL: https://codereview.chromium.org/11312205

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@167440 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build