fix(browser): check for fetch on window

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[codecov]

Codecov Report

Merging #1368 (c4d7152) into master (4b881d6) will decrease coverage by 0.01%.
The diff coverage is 98.54%.

@@            Coverage Diff             @@
##           master    #1368      +/-   ##
==========================================
- Coverage   98.53%   98.51%   -0.02%     
==========================================
  Files          32       32              
  Lines       19420    19444      +24     
  Branches     1368     1371       +3     
==========================================
+ Hits        19135    19156      +21     
- Misses        281      284       +3     
  Partials        4        4              

Impacted Files	Coverage Δ
dev/src/v1/firestore_admin_client.ts	98.77% <98.36%> (-0.05%)	⬇️
dev/src/v1beta1/firestore_client.ts	97.74% <98.46%> (-0.05%)	⬇️
dev/src/v1/firestore_client.ts	98.04% <98.70%> (-0.05%)	⬇️
dev/src/reference.ts	99.89% <100.00%> (ø)

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